I've seen a play through of the first two levels of the final Russian version of Clear Sky, and as a big Shadow of Chernobyl fan I can safely say that Clear Sky is "alright". By that I mean that, so far, it does fail at forcing a heavy and intense atmosphere on the player like SoC did, simply because there's indeed just too many enemies and just too much "life" around to say the least, for a post-apocalyptic and rotten zone of lethal dangers. In terms of atmosphere and spookiness, from what I saw, Shadow of Chernobyl is better. But there's also a very good thing in Clear Sky which I noticed right away, and it is the artifacts system. In Shadow of Chernobyl mere exploration never lead to anything worthwhile to justify the very possibility of doing it, other than finding a few treasures that were spawned in a scripted manner when you were looting dead corpses around. And the artifacts in Shadow of Chernobyl were scattered to some extent, but still far too easy to come by and it never felt "special" to get your hands on one of those.
That has been fixed and enhanced a hundred fold in Clear Sky for sure. Just wanting to go on a search for a single one of those objects should immediately result in a "buy all of the health packs and ammo you can find" reflex, because you may well need those items, for looking for them is one thing, but actually finding them seems not to be free at all, either costing lots of health, or lots of stamina due to having a horde of mutants running at you (that's where the "get more ammo" part comes into play). So getting one artifact in Clear Sky will certainly be a nice little side quest by itself whenever the player feels like doing it. The one thing I don't know about for sure is if those artifacts are still scripted and spawned at exactly the same place every time, the video I saw didn't "test" that of course, the guy playing was his first time as well I believe.
Overall (I'm skipping lots of stuff here) I'd say that just out-of-the-box Clear Sky is sufficiently good to justify buying it, it seems bigger (the first level, the Swamp, is BIG, I mean really big) although still limited by fences, indeed, than SoC was, and the new features are a welcome addition, the weapons upgrade system was something that would have made SoC even better, and Clear Sky has it, so that's a definitive plus. Let's not forget that Shadow of Chernobyl wasn't all that impressive under its unpatched vanilla condition when it first came out. But to conclude this I'd feel safe to say that, at the moment, Shadow of Chernobyl is still a better game when we take into consideration the patches that came for it but also its modding community who of course had a good year and more to make a number of very impressive modifications for it. But I would see Clear Sky as a winner given the same time for the community to work on it and the developers to fix the bugs and various mishaps. To put it simply, Clear Sky has more potential than SoC ever had, let's just wait and see how far it goes over time.