jlgx i recommed it.
It is not your average shooter. the atmosphere is second to none, gun fights are deadly, and the environment is deadlier. Story line is great, and has 6 endings, 4 fake, one wrong and the real ending. For memory. It can take as long as you want it too. if you run around gun ho like a typical FPS you'll die VERY fast. if you play smart, and be cautious, you wont die as often lol. And it is rather long just following the main story not the side quests.
The first game, S.T.A.L.K.E.R: Shadows Of Chyrnobal is the one you should buy. Clear sky, is the second game (although actually taking place before the first game) was dissapointing, and a computer killer. poorly optimised.
Buy SOC and you wont be dissapointed, IF you enjoy, being scared and having to THINK while playing games. It's not one of those games where you can horde 5 tonne of equipment in your backpack for instance. I found it a breath of fresh air, but some thought it was TOO hard.
EDIT: plus once you've finished SOC, you can then download the mod PRIBOI Story. which is a whole new game, new story, levels, mechanics, gameplay, monsters and all. Very well made mod.