I've used Kubuntu for a long time. In fact I had never used plain Ubuntu except for once a long time ago on an old laptop but then I promptly removed it because it caused the laptop to overheat badly (it was painful on the thighs) and so I went Gentoo, my bread-and-butter distribution. I never liked Gnome for some reason. I never had good experiences with Gnome in Debian or Gentoo. KDE always seemed to work a lot better out of the box. It might be because KDE is an all-in-one solution whereas Gnome is a bit more modular and made up of metacity, gnome-panel, nautilus and a bunch of applets. I remember trying out Gnome in gentoo and metacity wouldn't start, or it would crash spontaneously. Not Gnome's fault, but it's a problem that just would never have happened in KDE.
Ubuntu has fixed all that by focusing on Gnome and ignoring all other desktop systems and window managers and making it work great out-of-the-box. Gnome is nothing like I remember it being. Kubuntu allowed me to continue using KDE, which I was used to but unfortunately I think Kubuntu has fallen behind Ubuntu. The biggest factor here is probably Ubuntu's far larger user base. More users reporting bugs and more developers eating their own dogfood. 90% of all Ubuntu tips & tricks and how-tos on the web are for Ubuntu, not Kubuntu. A few things turned me off Kubuntu in the last while:
- Totally broken X config in KDE Control Center (mangles xorg.conf file badly, had to use nvidia-config or "dpkg-reconfigure xorg-server".
- Documentation on the web is almost always Ubuntu-specific and relied on going to menu items that just aren't present in Kubuntu (at least not out of the box).
- Not easy to set up Compiz on Kubuntu Gutsy (at least if you upgrade from Edgy). It's easy to set up in Ubuntu Gutsy or Hardy.
- Bad experience with KDE 4. I tried it a few times after it was released and it felt like a piece of alpha software. Was hoping for a better experience but it was buggy and I don't think Kubuntu has the resources behind it to perfect it like Ubuntu does with Gnome.
- Auto-mounting had been good in Kubuntu but once in a while I have problems (not able to unmount, devices spontaneously unmounting themselves, etc.... I have never had a problem in Ubuntu so far and I enjoy the fact that I don't get that annoying pop-up after I plug something in in Kubuntu or Windows.