Although I'm not quite sure about KDE 4.0 (going by the betas I've tried), I can't deny the look is definitely clean and polished.

This review was posted on 06/01/08: http://drowstar.blogspot.com/2008/01/first...de400-with.html

You can be sure about KDE4. All the developers wrote, at least those I read, that KDE 4 final though stable is just a technology preview. Many applications are not ported yet to KDE 4, many features are not ready yet. The first feature full release will be KDE 4.1.