I was very close to dumping Ubuntu. This has so far been the best Desktop Linux distribution I ever worked with, and I really liked the ease of use. But one day, something broke. It didn't allow me to switch between keyboard layouts any more! As I often have to compose texts in German, this is a big deal, and quite a problem. I was exploring the various forums for months! I found quite a few people who had the same problem, but nobody with a solution that actually worked for me.
Well, I finally managed to fix the problem. For whatever reason, the keyboard layout database got corrupted, and there is no straightforward way to fix it (e.g. by reinstalling a package). So instead I dumped the following directory: /etc/X11/xkb and replaced it with the same directory from a clean Ubuntu installation. That did the trick.
I was lucky that I had a fresh install lying around on another computer, but what a pain in the neck.