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).