S.T.A.L.K.E.R.: Clear Sky has not been released in America and most of Europe but some reviews of it are starting to surface.
Both, eurogamer.net and ign.com gave it a 7/10 and didn't really say good things about the game. The main complaint from these two reviewers is that the artificial intelligence is now too hard and that the artifacts are too difficult to get. They also complain that Clear Sky is as buggy as its predecessor.
As far as new features are concerned, the best additions to the game are the new upgrade system and the improved graphics. However, it seems that you won't be using the upgrade system much for money is something you barely have in Clear Sky.
Eurogamer
The problem is simply a string of design and balancing issues. In practice, after I received my first artifact in the training sequence, I didn't get a second for at least ten hours of play. That's "get", not "find", by the way. In some cases I was in the right spot - or, at least as close as I was when I managed to get hold of one - but completely failed to make the thing appear. In most cases, however, the area I was lead to was toxic above what I could survive for long enough to locate the tricky blighter without burning medkits. So I mostly just ignored it and carried on...Read more.
IGN
The S.T.A.L.K.E.R. community, reacting to advance word that Clear Sky isn’t the hoped-for triumph, are already saying post-release patches and mods will make it glorious. Yes, it’s possible, and maybe even probable -– bugs can fixed, balance can be altered. It’d be foolish to rely on it, however, let alone use it to vindicate the game’s reactorful of misfires. S.T.A.L.K.E.R. was made so much more of by patches and mods because it was such an impressive skeleton of a game. Clear Sky, though, has so much decaying flesh attached to it that cutting through to the admittedly very healthy bone below may prove too tall an order...Read more.
Personally I think those are very harsh reviews. They mainly emphasize on the negative points of the game rather than on the positive ones.