Picked it up yesterday, and so far, I'm enjoying it more then the first Stalker. Weapon handling is similar in that ranged combat is a royal pain in the arse till you get your hand on some rifles. Weapons/armour also have an upgrade system which is also pretty handy, with combinations of certain upgrades opening up further upgrades and excluding others, so you can either increase the accuracy upgrades at the expense of exluding your rpm upgrades. Some upgrades even let you rechamber the gun also, which I thought was a very cool touch.
Story is pretty meh, and apart from the beginning of the game, I haven't experianced the new Faction Warfare, but so far I'm enjoying it. I just wish that they used less of the original maps. basically the starting area is new, then you go to the Cordon where you started the last game, then to Garbage, etc. The day/night cycle is also pretty cool. Running around when you can bearly see 5 meters in front of you with shit howling away and the wind rustling the bushes is actually pretty creepy.
After 5 hours I give it

out of 7 if you enjoyed the original stalker (and expect more of the same). If you didn't like the original, you wont like Clear Sky.