I've been using openSUSE Linux for many years now, and never saw any compelling reason to switch to Ubuntu. There still isn't a compelling reason for the most part, but I will shortly get my new Nokia N900 phone, which runs Maemo, a derivative of Debian Linux. The Maemo SDK requires a Debian Linux to run it, and I want to run it, so I decided that was a good enough reason to move to Ubuntu.
I've had a couple of goes at moving to Ubuntu 9.10 "Karmic Koala". Along the way I also tried Debian Linux 5.0.3. I rejected vanilla Debian because the package management seemed weak. Linux distributions with the GNOME desktop typically come with the "Evolution" e-mail client. Since I use Opera for e-mail, I don't want Evolution wasted disk space. Under openSUSE, it is easy to uninstall the Evolution client (although you have to leave the "Evolution data server", as some packages have dependencies on it). With Debian, I couldn't remove the client or anything related to Evolution, which suggested bad package management to me.
Moving from openSUSE (using GNOME, not KDE) to Ubuntu was straightforward. You basically just need to copy your files across from your home directory, plus your files from anywhere you have installed your own applications (e.g. "/opt"). When copying your files
- you can use NFS, or tar to move your files (it's faster to just tar without compressing, if you have the disk space to do so;
- don't use Samba or ZIP files, as they lose important permission information.
After copying your files across, make sure your home directory has "rwxr-xr-x" (755) permissions. Also, in your home directory you should run the following to delete configuration files that can cause you problems (and which don't appear to lose anything important for your desktop):
rm -rf .config .gnome*
That's all there was to it. I haven't fully set up all of my applications yet (i.e. need to add some icons to the menu, etc.), but otherwise it's all looking good. While I've described moving from openSUSE to Ubuntu, I expect it should be similarly straightforward to move from Ubuntu to openSUSE.