I tried KDE 4 a while back (maybe last summer?) and didn't like it at all. I'm very conservative in terms of UI -- my KDE3 desktop looks very much like the Windows 95 desktop I used 10-15 years ago -- and all of the modern glitz bothers me.
That said, I'll probably upgrade to KDE4 some time soon... and then disable all of the distracting graphical features.
I tried 4.0 in a VM upon release, and that scared me away from the 4.x series until 4.2. Even in 4.2, I had a lot of issues because of my heavily customized setup. I'd say that if you're happy with 3.5.x, stick with it as long as there are no major security holes.
I got tired of basic functionality being broken even in 4.3, and left for greener pastures (Openbox). I only briefly experimented with 4.4, so I can't really comment on it (though from what I did see, I was not enamored at all).
well you can disable all the bling-bling, so not so much to worry. IMHO is the new version of konsole really nice, which is actually the KDE program I use the most.
That said, I'll probably upgrade to KDE4 some time soon... and then disable all of the distracting graphical features.