Have to agree with The Headacher on this one (not that I don't usually). I have both KDE 4.0 and KDE 3.5.8 on my system. I'm not a big KDE user, but if I had to choose, it would definitely be 3.5.x.
Right now 4.0 is sort of the "Vista" of *nix desktop environments, i.e. lots of promises and not much to show for it except it needs more resources than its predecessor. Having said that, it is a .0 release, and we probably won't have to wait 5 years for 4.1. The next release will probably be more stable, more fine tuned, and have a lot of of the customization options put back in. Probably. We'll see.