Installation is where this game falls down - be prepared to install it more than once before it works properly due to a custom and pretty rubbish installation system. You really must patch it to get the best from it but after that... M$ made a mistake - the Wow doesn't start with Vista, it starts with S.T.A.L.K.E.R.
System-wise, I'm running an unclocked Athlon 3800 single core with 2GB DDR and a lot of swap (2-4GB is gonna get used so make allowances). Graphics are great but require a VGA card with plenty of muscle (I've got an ATI X1950XT, also unclocked) if you want to see the best quality scenery and fx. Sound is mostly spot on, the odd misplaced voice or effect doesn't detract from gameplay.
Day & night cycles coupled with some fairly nasty locals (human or otherwise) can make life in the Zone harder than trying to make a soufflé in the middle of a riot. Be prepared for a fair bit of run & gun, sneak & stab, trading, conning & blagging your way through this nightmarish take on the Chernobyl saga.
If you get claustrophobic you might not like some of the underground areas - but if you want to survive in this irradiated mutant filled region of the world you'd better get over that real quick. Then again, being underground means you don't get caught out in a storm - and stay dry.
The game contains some of the most complex interactions between player and NPCs I've ever seen, side missions can span days of playtime (1 minute in-game = 3 seconds realtime) and earn you enough to buy better weapons, body armour and ammo. Or you could simply arrange for enemies to donate them to you. They don't like to, but in the Zone - that's life.
Doing favours for other Stalkers can earn you cut price deals, jobs or at least a reduced risk of being shot out-of-hand if you are dumb enough to point a gun at someone you don't intend to kill.
As I've mentioned, the Zone has a little problem with radiation; step into the wrong place and you can fry fairly qucikly - buy better shielding and you'll get on just fine. Usually. The anomolies can take their toll on the unwary and artifacts found can either help or hinder. You'll learn. Or not.
One thing I've found (having played the game to one of it's multiple endings for about the fourth time) - it changes. One run through it can be easy, another hard - your interactions with, say, the local military can have a bearing on how many bandits you meet later on. You learn to balance anger with a healthy aversion to lead poisoning, usually of the 9 or 7.62mm variety.
Overall, the only thing that caused me trouble was that ruddy installer - I've posted enough about it on the game's forum (along with many other people with major issues) and for this I give it 4 stars, but I'm glad to say that once I got my problems sorted out I've had a great experience with it and will probably be playing it for at least a couple more years (and by the time my hardware is the latest 2008 spec, at high rez with everything turned full on - it's 800x600 with all on as of May 2007)
I haven't even started playing multiplayer yet although I have looked around the supplied maps. For the online gamers out there - this will be *fun*. Beats seven shades out of Prey, Doom 3, Quake 4, any UT up to 2004 and even the previously untouchable quality of Half-Life 2 (after the bugs were worked out of it).
And no, if you were wondering, I have no connection with anyone connected with the game even though I may sound like a shill. I pre-ordered this game from Play in 2004 and now I'm playing it, I'm happy. Not bad for someone in his mid-forties. If you've read this far without going nuts, thanks - and I hope you enjoy this game as much as I have done.