Please temper your expectations. I really enjoy the STALKER games but they are no where near as mind blowing as people here claim them to be. It's one of the most ridiculous circlejerks ever.
The games have no real survival element whatsoever (You don't need to eat, drink, or sleep, ever), they aren't scary at all, since you are armed to the teeth, practically invincible with a good set of armor (I never even use first-aid kits anymore because I have great armor and artifacts which heal me) and mutants are easy to kill.
The only scary enemies are the cloaking bloodsuckers, they'll occasionally will give you a jumpscare, but they're easy to kill otherwise. If you've played a game like Amnesia, STALKER won't even slightly raise your heartbeat.
The combat AI is dumb but has superhuman accuracy and reflexes, the peaceful AI is also not nearly as advanced as people claim.
The AI has no really interesting "purpose" for what it's doing, they came the closest to having a purpose in Clear Sky where the AI factions are attempting to push the other faction back to their main base, the AI has no strategic processing, they just mindlessly pursue certain checkpoints in a given order until they reach the enemy base. The AI in STALKER is no more advanced than a game like Fallout, where they are pretty much just running on a schedule between checkpoints.
The world they setup is interesting, but they never did anything good with it or expanded on it. Everything is a macguffin explained away by a Deus Ex machina, it's pretty much bottom of the barrel story devices. The faction ideals are all vague and not fleshed-out, and they never do anything in game towards these goals, all they do is just kill the faction with the opposite viewpoint.
The STALKER games are good fun, but they aren't scary, they aren't hard, and the story is intriguing, but was poorely handled on the whole.