My main worry prior to playing was that the game wouldn't manage to successfully drag out it's only-one-enemy scenario to a decent length without getting boring. Happily those concerns were allayed. The game throws in enough varied scenarios to keep it feeling fresh (as well as the big twist just over halfway through that reveals there are more than one Alien). It also mixes it up by making its Seegson androids a serious threat later in the game, with the added bonus that they make for very different opponents to the Alien — they're less immediately deadly, they're less mobile, and they're slow and lumbering in comparison, but they are relentless. What makes it even more amazing is that you know you could easily outrun them by hitting the sprint button, but if you do you'll likely draw the attention of the Alien, and then you're dead. So chases turn into lengthy walking-pace foot pursuits, as you desperately try to build up enough distance between you to reload your weapon or slip unseen into a hiding place.
Another great trick the game pulled to vary its play style was the fact it gets more action-packed (at least by this game's standards) towards the end. The Aliens become markedly more aggressive, and the game forces you into situations where sneaking around is no longer an option. You have to push through them, using your flamethrower to scare the creatures off whenever they appear. The knowledge that you have no choice but to use your only real weapon against them, even though doing so will only make them more angry, while at the same time you're starting to run out of fuel, makes the final act truly pulse-pounding.
Sevastopol itself is one of the best things about the game. Not since the original BioShock have I played a game with such a well-realised, lived-in world that feels so believable. The design aesthetic is spot-on, especially in the grimy engineering levels, which look almost exactly like the lower decks of the Nostromo. At the same time the game comes up with it's own look for much of the station, so it never feels like a slave to the film(s) that came before it.
Any complaints I do have are minor. The Facehuggers are rather annoying, being one-hit-kill enemies that quite often take you down before you even knew they were there. Also the game suffers a little in the replayability department — it's still great second time around, but nothing matches the terror of the first time you went through it. The "easy" difficulty is also something of a cakewalk; hiding from the Alien's is a walk in the park, and it barely even shows up at all unless you do something to attract it. If you're taking your first shot at the game, I recommend "hard".
The only thing left is survivor mode, which I've yet to really try out, and the bonus pre-order levels, which I still haven't attempted. But the main story is great, with interesting if underused characters, an engaging protagonist, some great set pieces (the first time through the medical centre and the Hive are definitely the highlights), a truly lethal opponent and a good campaign length. Definitely one to try out.