Extremely good. I just picked up Pripyat myself as it's the only one of the series I don't own, but Shadow of Chernobyl is easily one of my favourite games... well... actually I'd go so far as to say that with the Complete mod to patch it up, it's one of my favourite games ever made.
It's just so damned atmospheric, there really isn't a single other contender made within the last ten years that even competes with the sheer amount of ambience this game exudes. Everything from the bored stalkers sitting around a campfire playing the guitar to the absolutely thunderous roar of lightning and the terrifying darkness of a stormy evening. The howls in the distance, the pearls of gunfire, the inky crawling blackness of a single warning light in an abandoned tunnel.
Just so brilliant. It really, really is. Gameplay is alright, the story is alright, the interaction is alright, the AI is surprisingly good. But the atmosphere is just so phenomenal, you've really got to experience it to know what I mean.
The first time you're stuck out in the dark during a thunder storm, barely able to even see a few feet in front of you with your meagre flashlight and flinching at the sounds of distant howling as you frantically turn one way to the other to try and find the source... then you'll understand.
Really do recommend the Complete mod for playing it as well. Acts as sort of an ultra patch, fixing and improving without actually changing. I love it!
Many people I know and trust say that Call of Pripyat is also extremely good, though the second game was a bit disappointing. But having not played it myself save at a friend house I can't say for certain.
Chernobyl though, brilliant. Worth owning.