Since I've bought it and played it today, some first impressions:
Clear Sky takes a few elements from Shadow of Chernobyl, expands on them and improves them and this results in a game that is definitely STALKER, but has a quite different approach. The elements:
- Engine: the engine is seriously improves and optimized. One can notice that very quickly by playing on the same settings one had for SoC and seeing the improvement on frames per sec, while having a general upgrade in the graphics department. Animations are way better, shadows/sun/moon have been improved and there are some minor changes in texture etc. making everything look smoother. And again, this is at the same setting as SoC (full dynamic without improvements) with better fps.
-Shootering: Clear Sky clearly choose to improve on STALKERS (already satisfying) gun mechanics. Guns feel more real and this is mostly due to the increased smoothness everything seems to have. Everything in SoC felt choppy, laggy and stiff, and CS makes it more of a smooth ride. This is probably mostly due to the optimized engine, but there are some minor changes in the gun mechanics. Also, there is a repair/upgrade system for armor and guns which is both nice and easy.
-Atmosphere: This is one of the areas CS seems to have chosen another route. Where SoC was at times terrifying and depressing, CS seems more action oriented and crowded. I don't expect any of the suspense filled underground areas based on my first few hours. That said, the atmosphere of the Zone's outdoor areas is still there and is hugely improved due to the graphics update. Also, the first level, the Swamp, is simply brilliant.
-Faction war: This is one area that was almost nonexistent in SoC, but has seen a major revamp in CS. Indeed, it seems to have been the focus of the game. There is a faction balance page on your pda, the game revolves around capturing points for your faction and this almost resembles Battlefield in it's setup. But there still is the exploration and huge open areas which made SoC great.
In conclusion, if you liked the open areas and shooter mechanics of SoC, CS is definitely a game to buy. If your main interest was the survival horror feeling of the underground levels, I think you should think about not buying this game.
EDIT: before I forget: I haven't seen anything remotely like the horrible respawn in SoC.