Seven long years in a development cycle fraught with public and private difficulties of epic proportions had convinced the industry that the original S.T.A.L.K.E.R. was vapourware. Like Starcraft: Ghost and that old hog Duke Nukem Forever, GSC Gameworld's much-anticipated debut in the first person shooter genre was often thought lost and on several occasions even declared dead - not least when file-sharing networks shopped around a pre-alpha build of the game four years before its eventual release. Now, a little perspective might useful here: four years is the equivalent of two typical AAA game development cycles, or the time it took for one once-troubled team to dream up, say, Gears of War, make a success of it, dominate the marketplace with One Engine to Rule Them All and then polish off a sequel that looks to be - indeed - bigger, better and more bad-ass; and that's just the time that this ambitious Ukrainian studio spent on S.T.A.L.K.E.R. after the infamous leak. So the widespread scepticism that the game mightn't see the light of day was hardly unwarranted. But in March of last year, Shadow of Chernobyl cast off its unfortunate legacy and arrived at retail to prove everyone wrong.

Well, almost everyone.

Actually, pretty much half of everyone - that is, if you consider everyone to mean gamers and only then the endangered core of FPS fans who hadn't yet swapped out their expensive rigs for a next-gen console. Half of this group absolutely adored Shadow of Chernobyl, including our own Ross Alexander who "loved every single challenging, compelling, atmosphere-laden moment" of it, and let me slip out of character for a second to say: body and soul, I agree with him. But while we were hardly on our own at the S.T.A.L.K.E.R. love-in, there was some pretty staunch opposition, because the other half of everyone - don't think I'd forgotten them - were moved in rather the other direction. Instead of the revelatory experience that Ross and myself and so many other players had, there sprung up another, equally vocal camp who would have been overjoyed to see Shadow of Chernobyl disposed of as though it were, like the ill-fated reactor meltdown at the core of its story, radioactive.

It admittedly had its fair share of faults - and perhaps a couple of other games' fair share as well. The patented X-Ray engine that GSC Gameworld built would have been a solid foundation for the game if it had seen release in a more traditional timeframe, but by the time of the actual launch it was already archaic. The graphics would have stunned you if you'd had the computer to run them maxed out, but in the end, only the developers did; and what little localisation had been done for the Western release of Shadow of Chernobyl was uniformly awful. The narrative itself descended into some unfortunate territory after an incredibly effective start. For instance, setting the player loose in a vast, desolate landscape devastated by the poisonous fallout of the accident at the eponymous power plant was a masterstroke; affiliating that tragedy with a series of Soviet experiments into the supernatural, however, not so much. It all seemed a little too Return to Castle Wolfenstein for such an otherwise-authentic experience and the story's hiccups only got worse. In quick succession there was a hive mind, an identity crisis, a monolith and plenty else besides in GSC Gameworld's little shop of post-USSR horrors, all of which sounds just dandy but at some point the Shadow of Chernobyl narrative started to sound like the half-crazed ramblings of a first-gen sci-fi fan.

Above all, though, the main complaint was that you couldn't travel far around its gargantuan environment without bumping up against a few bugs and, eventually, one or another of them would stop your tentative progress through the Zone in its tracks. For some reason, a bunch of people had a problem with that and, without being dismissive, I think it's fair to say that we all have to learn about staged game saving somehow. All stilted attempts at humour aside, if you could learn to live with the game-killing bugs, a few dodgy accents and the apparent lack of optimisation or any kind of quality assurance, there lay beneath them a world so oppressive, so unwelcoming and so alien that it became, by some strange Soviet alchemy, utterly human and absolutely compulsive.

Shadow of Chernobyl was something of a love it or hate it affair then, with precious little middle ground betwixt the two; like Silicon Knights' Too Human after it, it was a unique take on a brilliant concept brought down by poor testing and, believe it or not - although an age in development tends to suggest the latter - release at retail before the game was good and ready. You've got to give it to GSC Gameworld, though; for all their shortsightedness and for all the mistakes I'm sure they'd admit to making, there aren't many other first person shooters that have so effectively divided the player base. The first S.T.A.L.K.E.R. presented such a startlingly original vision that scores of gamers fell for it, despite its innumerable faults, and sometimes, perhaps, because of them. To return a moment to AceGamez reviewer Ross Alexander's reasoning for his very positive score: "It's just so different from everything else that's out there. It may be flawed in little ways and lacking in the gloss you'd expect from the likes of Doom 3 and Call of Duty, but that's just part of its beauty." The grim foreboding of the atmosphere, electric with terrifying possibility; the leftover Russian dialogue; the tedious inventory management system and the hour-long treks to long forgotten quadrants of the remarkably true-to-life exclusion zone. And who could forget the apparently purposeless buildings, the inexplicable obstacles you encountered as Shadow of Chernobyl's amnesiac protagonist was pulled inexorably to the perilous power plant? The world, the combat, the presentation - every aspect of the game seemed composed of unappealing parts, but together the experience gripped tight and did not relent. Unpolished and unforgiving it certainly was, but compulsive for all that.

Nonetheless, S.T.A.L.K.E.R. take one was riddled with so many bugs and elementary oversights that when it finally arrived on store shelves, critics were wise to recommend the game with a word of warning and an optimistic eye to the future. The vast majority of Shadow of Chernobyl's faults were indeed patched away - and you can be sure that GSC Gameworld will fix up Clear Sky in much the same way - but by that point the barn door was closed and the horse had long since bolted. Internal testing is hardly a substitute for the vast spread of feedback that developers get after launch, but poor QA can kill a game stone dead; if your product is more trouble than it's worth to play in the first place then who's going to bother sending bug reports? Luckily, Shadow of Chernobyl shifted more than two million units worldwide and considering that this year's prequel is more of a standalone expansion than a second instalment in the series, you'd be well within your rights to expect that GSC Gameworld could more easily streamline the bug-stomping process. Unluckily, you'd be mistaken. Mark my words: if you go down to the Zone today, whatever you do, don't go expecting a big surprise. Keep your expectations in check and Clear Sky can be the equal of its divisive predecessor, but go in with superlatives on the tip of your tongue and you will be disappointed. Day one, it's just as buggy, if not more so. The story - which has a veteran Stalker by the name of Scar venturing to the heart of the Zone to investigate a series of mysterious emissions - is just as ridiculous and worsened by some slipshod localisation. Although it's still scattered, there's a great deal more English voice-work this time around, with no option to switch back to the original Russian that lent the narrative more credibility than it was perhaps due last time. The rig you'll need to see Clear Sky at its best is still a few years away from being acceptably priced and, despite an overhauling, the graphics are much the same as they were last time but with an all-new, all system-stalling array of bonus volumetric wotsits.

The most overwhelming disappointment isn't any of the above, however; it's that in futzing with the delicate balance of activities that a Stalker undertakes during a typical trip through the Zone, GSC Gameworld very nearly breaks the vital filament that made Shadow of Chernobyl such a standout: its atmosphere. There are a multitude of culprits here but foremost amongst them are the fleshed-out faction wars. As story missions carry Scar further from area to area, he encounters six different factions that are at war with one another over territory and ideology. From religious zealots to military types, all the usual suspects are accounted for and, as the environment opens up, you can ally with any of them, helping to take over valuable locations across the map and fend off the enemy as and when they attack your camps and bases.

On the bright side, the faction wars extend the best-before date of Clear Sky immeasurably; at around twenty hours, its campaign is hardly trifling but you can easily spend a few hours in each delineated region of the Zone. From the broken-down hovels of the labyrinthine swamps where you begin to the fireside camps and radioactive rubbish dumps in the Garbage - and at nearly every location thereafter - there's the opportunity to put your familiar arsenal to good use, and if you're game enough to follow the instructions delivered to your PDA then you can in many ways tip the balance of power across the post-nuclear landscape. Your PDA also comes equipped with a handy little chart to help you keep track of any allegiances and a map with colour-coded markers denoting the movements of any parties in the surrounding area - even horrific packs of dogs and other mutated creatures bent on tearing you from limb to limb are deftly described as 'mutants' or 'bloodsuckers' for your convenience. All of this rather takes the terror out of Clear Sky; the unknowable horror that could lurk around every corner of the Zone before is now clearly signposted and ultimately reduced to handy little dots on your GPS mini-map. The other atmosphere-sapping aspect is the higher encounter rate; whereas in the original game you could feasibly spend hours sneaking around the Zone without bumping into a single other soul, bravely overcoming the climate of fear to hunt for a few stat-boosting artefacts or check out some of the loot stashes you've heard about, here there's a monster seemingly around every corner. Clear Sky positively teems with life and if Freedom and Duty aren't harassing you to come help shoot one another then you're being hounded by vicious pseudo-dogs; you'll know because that's when the asynchronous action music loops in, destroying any kind of accidental build-up of tension.

You can turn the ghastly battle theme off, thank God, and even tune out most of the faction war nonsense, but at your peril - there are set-pieces later in the game that are considerably more difficult without the choice pieces of kit that factions tend to welcome you into their gang with. Ultimately, though, there's nothing to be done about the overpopulated environments or the endless ruckus that the carbon-copy armies kick up, and all the noise amounts to a decidedly less atmospheric world. Sure, there's more to do now, and the world feels more alive - it should, after all; don't forget that these are the areas outlying the Chernobyl reactor before it melts down the second time - but the experiential price of that narrative contrivance is staggering: the Zone just isn't very interesting any more. It's no longer low budget foreign horror movie scary so much as Saw the Sixth, and the vast increase in encounters underscores the deficiencies in the difficulty curve and the competency of Clear Sky's AI that could otherwise have snuck by with a slap on the wrist.

We're at a point now, plainly, where the level of artificial intelligence needed for a first person shooter far outstrips the skills of even experienced humans; the contention now is in teaching the bad guys and squad members how to be authentically imperfect. As in Call of Duty 4: Modern Combat and Battlefield: Bad Company, let off a shot in the Zone and every enemy in your line of sight will rattle a round off at your armour-plated gut; to make matters worse you'll find the grenade throws of Clear Sky's cannon fodder astonishingly accurate - even after the latest patch. They're not above outright stupidity, either; some of Scar's marks take cover right between his crosshairs, or line up to learn one last, lethal lesson. The push in AI now is towards a level of intelligence more closely resembling our own, a more fallible subroutine; and on that count, move along, there's nothing much to see here.

Except that you won't be moving along - not soon, anyway - because even on the lower difficulties, Clear Sky routinely sets you against impossible odds and impossible enemies. You can unload entire armour-piercing clips into later enemies - six automatic-shotgun rounds at close range, say - and still have to hide and heal in order to be ready for round two. This isn't to say Shadow of Chernobyl was easy - far from it - but in that instance the difficulty was better moderated. The AI wasn't so downright perfect and there wasn't such a focus on one-man armying through a wall of bad guys; surviving was what counted and if you were smart then you could manage that without mummifying yourself in bandages and sucking at health kits for dear life, all whilst mashing the quick save key. You had to be ready, that was the point, but you weren't expected to fight to the death ten times just trying to get to the nearest merchant.

If you own a PC to be proud of then Clear Sky has the potential to take your breath away. With death and destruction all around you, the sun finally breaking after a hard night's hunt is exceptionally satisfying. The Zone ripples and Scar with it as emissions approach and for a fraction of a second the world is viscous, pliable. Whether you've got God-rays on or not, brilliantly understated art design tempers the post-apocalyptic horrors with a fragile beauty that undermines your expectations. Putting aside the unconvincing battle theme and some truly awful English language audio, the sound impresses too, although much of it is borrowed wholesale from Shadow of Chernobyl - even the title music. There are occasional performance issues, from drastic frame-rate drops in heavily populated areas to the occasional crash, but technically Clear Sky scales pretty well; you should get a reasonably consistent 60 fps from a mid-range system on medium settings and anything from two to twenty with the volumetric sliders cranked up. In any case, the game is still prettier than almost anything you'll see on a next-gen console and many of the most offensive bugs have already been patched - although you should be advised that the latest, as of this writing, will corrupt your save files.

This might seem like a laundry list of problems and if you're among the metacritic masses who've already skipped ahead to look at the score then you must be wondering when exactly I'm going to get to the things that Clear Sky does right. Especially for you, then, let's just say that everything else about this game is pure S.T.A.L.K.E.R. That's somewhere between Fallout and The Elder Scrolls with a spurt of Silent Hill; bleak, ruthless and punishing. It's an obstinate game, riddled with silly mistakes that'll be updated into oblivion eventually and filled to bursting with apparently backwards design decisions that force you to do things like hide in shacks littered with bodies that you didn't put there for five minutes at a time while emissions turn the world funny colours. It's in those moments that Clear Sky is at its best. It's when the day/night cycle kicks in while you're amid a minefield of anomalies in the Red Forest and you realise that that the next hour of playtime will be by an impotent beam of torchlight; the monsters are still waiting for you and now they can see you coming. Whether the developers stumbled onto this beautifully twisted thing by accident or by design is immaterial; they did. Sadly, Clear Sky is a visibly compromised product beside its predecessor and it cannot scale those same heights - yet there is no shortage of reassuringly standout moments. At the very least, Clear Sky is proof that lightning can strike twice; the startling vision that so nearly came together in the first S.T.A.L.K.E.R. is GSC Gameworld's for the taking back. Now let's see if they can run with it for the inevitable full sequel.

Reviewed by Niall Rough for AceGamez (All Rights Reserved).