S.T.A.L.K.E.R. is one of the most atmospheric, realistic, befuddling game I have ever played. The game just sucks you in. It had a few *cough* glitches when it shipped, but nearly all of them are fixed. I played it again recently, with the patches, and I didn't run into any problems.
So yeah, get it. It has the best ballistics ever. Bullets ricochet, get moved by gusts of wind, dropped by gravity, take time to get really really far away (Don't expect your pistol to get 10000 meters in a split-second). All in all, it is an excellent game.