I've switched both our laptop and desktop/server to Kubuntu 9.04. Great, great distro.
Boot time is fast. Really, fast -- maybe 30-45 seconds. And system is very responsive given the change to the ext4 file system and KDE 4.2. Stable, and runs everything. Devices found and drivers loaded out of the box (just had to enable the 3rd party ones). No problems found whatsoever.
The cutover from Mandriva 2009 to Kubuntu was pretty seamless. I actually wiped the /home directories in favor of reformatting with ext4 from ext3 and recovered them from backups.
Best part about Kubuntu: so many people are using (k)ubuntu that all problems found so far are solvable with a quick google search.
http://www.kubuntu.org
