Now that kde 4.0 is out for one week, I'll try to make a short summary of my experience with it: + installation (using kubuntu gutsy) is a breeze + coexists nicely with kde3.5, uses separate path (/usr/lib/kde4) as well as personal configuration (~/.kde4) + import preferences (e.g. konqui bookmarks) from kde3.5 is easy + little quirks/annoyances get fixed at astounding pace + kde3.5 apps run nicely within kde4 + kickoff menu is impressive + system settings much better organized/cleaner than kde3.x's control center + krunner is much more informative/flexible than previous 'ALT-F2' + building cmake-based kde4 apps from source (e.g. from kde-apps.org) is much more convenient than the old auto* stuff (crap) - desktop icons often cluttered/duplicated upon startup - background picture gets lost upon startup (already fixed) - 'show desktop' (CTRL-F12) doesn't show systray/taskbar - many things (e.g. taskbar size) are not (yet) configurable - some annoying error-message popups (cannot start xyz.. - but starts anyway) - very poor performance on (re-)drawing screen (parts), this really drives me nuts, as I'm under the impression I'm sitting in front of my ancient AMD K5/300 MHz back in the kde2.0 times :-(.
and no, I don't use any desktop effects/3D stuff or whatsoever... just plain xorg/intel driver (kde3.5.8 beeing fast/snappy as hell with this setup on this Dell 2.4 GHz machine).
So, all in all, somehow meets the expectations, one could get from all the blogs/comments beforehand on planetkde.org. I just don't use it as my working DE for now because of the mentioned screen drawing performance (or the lack thereof).
I hope, this gets addressed/solved soon (which I have no doubt about, looking at the speed of improvements, as they are done).
For now, I'll stick with my beloved kde 3.5.8 DE, but keep an eye on 4.0 regularly - ready to switch over when it gets usable w.r. to performance - I know most people out there who have tried 4.0 don't suffer from such weirdness, so for anyone willing to try: just do it !