I've used a few different Ubuntu releases, from Karmic Koala to Precise Pangolin. They are a mixed bag (but Precise is AWESOME.)
Karmic Koala - I didn't like this much, screen flicker was a major problem for me. Having to edit grub.cfg every time I downloaded a kernel update - to not entirely deal with the problem was a pain.
Lucid Lynx - Not bad. Same problems as above. KDE had the awesome Bespin style available. (I miss it...)
Maverick Meerkat - Often crashed during shutdown. Irritating.
Natty Narwhal / Oneiric Ocelot - When booting the more recent kernels, the screen would remain completely dark.
Precise Pangolin - No problems except NetworkManager in KDE (but I use WICD anyway).
"Zero zero zero zero zero zero one! Zero zero zero zero zero one one! Zero zero zero zero one one one! Zero zero zero one one one one! Oh, oh oh one, oh oh one! Come on sucker, now lick my battery!"