It took me along time to switch from KDE 3 to KDE 4. I finally upgrade with KDE 4.5. Before that, things crashed. I wouldn't recommend anyone KDE 4.0 ... 4.4.
Since 4.5 IMO it's rock stable and beautiful.
I didn't even switch the wallpaper or the window decaration, what I always did before.
It just looks beautiful and is more configurable than ever (if you want to).