I have had ubuntu and Kubuntu side by side on all my computers for years.
KDE 4.0 on Intrepid was painful, but since Jaunty I have had few, if any, problems.
I use KDE 99% of the time, now.
KDE on Mandriva or OpenSuse (or others) may be a bit more stable (I haven't tired them), but the Ubuntu team works hard to integrate Gnome and KDE applications into the same repositories and to enable cross-platform compatibility. Is that being done in Mandriva or OpenSusE ?
I do use, even in KDE (Kubuntu) a lot of Gnome applications, so the cross-platform compatibility is important to me.
After reading this thread, I intend to play with LXDE further, since I have heard so many great things about it.
(However, I had heard a lot of great things about the Xubuntu desktop, Xfce, and that was a total dog.)