I've now had KDE 4.0 installed for just over a week, and I'm getting used to its quirks. I'm very impressed with KWin - it hasn't crashed on me once. At least, not since I got the right combination of options (for me, the intel graphics driver instead of the i810 one, and OpenGL instead of XRender). And it's a darn sight nicer than compiz. I always got frustrated by compiz because it didn't understand KDE properly, and I couldn't use Alt+RMB to resize and things like that. And it would crash. KWin just works, although rather slowly.
Generally, KDE 4.0 seems a fair bit slower than KDE 3.5, but I think that might be down to the compositing stuff. Also, there are quite a few little niggly bugs which I hope will be fixed by 4.0.1. I've reported a few bugs, but I'm still trying to work out reliable ways of reproducing some of them before I report them.
I said in my last post that I was inspired to start coding for KDE 4.0. Well, as usual, my inspiration has quickly waned as I moved into a new house and have more useful things to do at the moment, such as painting. So don't expect code from me any time soon.