I remember playing through STALKER. I grew tired of the aiming and installed a mod for heightened accuracy. That increased my enjoyment of the game tenfold. From there I installed a few other mods to make the game harder to offset the accuracy increase (such as allowing enemies and animals go anywhere they want, rather than being limited to specific zones). It was a very fun game, with an interesting story.
My favorite thing about it though was their use of sound and visuals. They did a really good job in creating a type of sci-fi/fantasy setting, especially in the later areas like Pripyat and Yantar. I especially loved the area where the mind ray thing (name escapes me for the moment) was. I had my bass turned up so everytime the towers emitted their "rays" my entire room would vibrate. Really pulled me in.
As for Clear Sky, I heard a lot of bad things regarding performance and glitches when it launched, so I decided to wait and see. Now I'm waiting to see if the publishing team ends up adding Clear Sky to Impulse. If not, I'll purchase it myself later.