I got into this game pretty late. It kept being mentioned as one of the best games of the year on game sites and everyone seemed to be pretty excited about the upcoming prequel. When it popped up on STEAM I bought it and then didn't play it for ages.
Initially it seemed to me to be a bit clunky. I wasn't wild about the human animations or the lack of talking over the external conversation parts (it seems cheap to have voice acting in some places but not everywhere you would expect it). Finally, there seemed to be a lot of running around and you start off extremely underpowered so combat is a little unsatisfying at first.
I decided to give it another go after a rave review from a friend in work and, apart from work and sleep, didn't stop playing until I had completed it. I should have hated it because it has a lot of backtracking, because it is tremendously buggy on my hardware (common but random crashes when transitioning between game zones and some odd event trigger problems) and because it takes a while to figure out the gameplay mechanics. Instead I found myself being really sucked into it.
The first and most important thing is that the atmosphere (especially graphically) is fantastic. The scenery is full of rusting machinery, decrepit buildings and overgrown flora. It looks very much like the pictures of Pripyat and the rest of the zone that I have seen which is apparently not surprising since the developers were able to gain access to it to do research. Your fellow stalkers and the various friends and enemies you meet all look realistically kitted out for the difficult terrain.
Then there's the gameplay. It's an interesting mix of reasonably freeform exploration/side-quests and a clearly delineated main path (the goal changes at a few points). You perform the side-quests to explore the various areas in the zone and to accumulate guns and artifacts (which bring special powers). Because of this the side quests never seem ridiculous and occasionally they are particularly good fun.
The main quest draws you towards the reactor building itself. Somewhat annoyingly it's possible to miss the main ending by not performing what appear to be a couple of side-quests earlier in the game. That main ending is fantastic, taking you all around the exterior of the reactor complex to a strange final showdown.
The AI is excellent throughout and the weapons behave very realisitically (with excellent modelling of ballistic drop and substantial spread on almost all weapons). Juggling armour, weight of inventory and availability of ammo adds a lot of useful difficulty to the game. It's certainly no pushover, even at the end when you are pretty seriously tooled up.
Finally, I think the thing that appeals to me most about the game is the careful reveal of the back story and the source material. It's based on the same source material as Tarkovsky's film (though it keeps much more closely to it than he did) and it was great recognising elements of the game (especially tonal ones) that I remembered from the film.