As you probably know, Kubuntu is an official derivative of Ubuntu, which uses KDE as it's graphical environment instead of Gnome, present in Ubuntu itself. They are both maintained by Canonical.
Canonical has chosen Gnome thanks to it's simplicity, ease of use and it's famous Human Interface Guidelines, which should make most users satisfied.
Personally, as a technology professional, I prefer KDE over Gnome. This is not intended to start a flamewar, as there are thousands over the Internet. It's just a personal opinion.
But Canonical does supports Kubuntu, some might argue. Yes, they do. But their focus is Ubuntu (and therefore, Gnome). Kubuntu is a second class citizen. No one can denies it.
The Kubuntu team has always been trying to catch up with Ubuntu's new features, and many times they are only implemented in the next version (for example, the restricted drivers).
Also, Kubuntu tries to be on the bleeding edge, but many times pushes incomplete features into the release. Some examples:
- Kubuntu Hardy had strigi and dolphin in KDE 3.5.x, which were both unfinished and looking rough.
- Kubuntu Intrepid had broken the bluetooth support
- Kubuntu Jaunty comes with a network manager applet with a bug that prevents it from connecting to wireless connections with a hidden SSID, and comes with KPackageKit for installing packages. The problem with it is that it doesn't understands debian dialogs, so, try, for example, installing sun-java6-jre package. It will silently fail, because you haven't accepted the license agreement, which wasn't shown because KPackageKit don't understands it. To fix this, I've installed the good old synaptic.
I think Kubuntu should be either an officially supported product or a community supported product. What disappoints me is this "theoretically official, but community in practice" thing.