I can't answer the "versus Kubuntu" part so I'm not sure how happy this comment will make you, but I've been using Fedora KDE since fall 2009 and personally hav

I can't answer the "versus Kubuntu" part so I'm not sure how happy this comment will make you, but I've been using Fedora KDE since fall 2009 and personally haven't had any problems with stability. In general I'm quite happy with how Fedora distributes KDE. They don't patch it very much and leave it mostly vanilla, focussing their work on system integration instead of modifying to differenciate their offering. That's reflected in the work they've contributed upstream, like PolicyKit support or PackageKit integration for widget installation in the desktop shell (for automatic dependency installation). Nothing flashy, more under-the-hood work, but somebody has to do that as well. Their KDE packaging team is very competent, and the leader, Rex Dieter, is one of the nicest fellas I've encountered in FOSS. Others can be a bit stubborn at times, but are still useful and supportive, making #fedora-kde on Freenode one of the better user support channels I frequent. Very nice for me as an upstream developer is also that they explicitly strive to support KDE development, so they will generally swiftly package any new dependencies required by KDE's development branches.