I tested KDE 4.0 in Kubuntu 8.04 Remix edition. I know it's probably not the best distribution to test the relatively new KDE 4.0, but that was what I had available at the moment. It didn't made a good impression to me though: it seems unpolished; I had to resize and adjust widgets nearly in every application I used; it's slow - one of the reasons I've left GNOME behind was the fact that KDE3 had a more responsive interface, which doesn't lag when you scroll through a list of, say 1000 items - well, that is not the case with KDE 4.0, which now seems as slow as GNOME is. I'm currently using KDE 3.5.9 with Debian Lenny and I'm so happy with it, and some of the reasons which still make me stick to it are:

1. Very stable
KDE3 has been tested and tested over and over; most of the applications for it reached maturity and are bug-free

2. Polished

More Here