I've only played the first STALKER, so if you're talking about the second one (Clear Sky) then disregard this post.
I bought and played the game about a year after it was released, and the version I played was pretty stable, but not perfect. I had several CTD's (most of which occurred while loading a new area), and there was one area where if you failed to save someone's life, you couldn't continue the main quest at all (and the game fails to inform you of this somewhat important fact). This happened to me initially, but I had several previous saves and just reloaded one of those.
Obviously, save often and in several different files, and your frustrations should very limited.
As far as performance goes, I think there was only one area where my framerates dropped significantly, but everywhere else was fine. And that one area had a ton of shadows, lightning, rain, and a perpetual gun battle. It was easy enough to bypass, and if I wanted to fight the battle, sniping from the perimeter (my preferred m.o. anyway) kept the framerates up high.
Overall, the game was well worth those minor annoyances. I played the Vanilla version, but I know there are several major mods out there that enhance nearly everything (and I assume stability and bug squashing), so you could also try one of those. But as I said, even with plain ol' Vanilla, I liked the game a lot, and would recommend it.
