Just about the best FPS game series there's been and this is the best so far. You want atmosphere, you got it.
Later patches eventually made the games a fantastic, tense, true sandbox, explore at your own pace scary world to get lost in. they also Have a fantastic MODDING community which adds to longevity.
I'm thrilled to say COP seems bug free and is the best of the series yet.
The only FPS that has come close to this game is the great Fallout 3. They are different but both should be at the top of your shopping list.
In STALKER COP you start off in the middle of a wasteland with a some basic mission objectives and have to explore, talk to lots of people, find things, scavenge and trade to progress and of course not get killed in the process by the many hazards, anomalies, baddies and beasties that live there. No mindless run in and kill everything linear experience here. Lots of side missions to do as well.
To upgrade your kit you have to find the guys who repair stuff anf then find them tool sets to allow them to do better upgrades. Plenty of weapons and kit to find or trade. With head scratching choices to be made on which to keep, upgrade and also which upgrade path to take for a particular item. You have to pay for stuff you dont salvage or find and it all needs repairing with use and damage which costs even more dosh.
Positives: True open sandbox, Great ballistics, Very very atmospheric and scary especially at night or in dark places, You have to think about what you are doing or your dead.
Negatives: Story line was weaker than original, Wished there had been bounty payments for killing mutants (one of the better MODs available on the original)