I am a full time Ubuntu user. As distros go, I’d refer to it as a love/hate relationship most of the time. Generally speaking, I am very happy with it. But there are a lot of people that have been asking me about the prospect of using other great distros instead of using Ubuntu. My response as to which, is that it depends on your comfort level with Linux.
Advanced users are encouraged to check out Fedora 14, as they made a lot of cool security upgrades worth checking out. And if you prefer to stick to a FoSS mindset, Fedora has a decent history here as well. My second choice, would be Debian. Again, a favorite among advanced FoSS enthusiasts.
For less advanced users or those who simply prefer things work with any available method possible (proprietary drivers, Adobe Flash, etc) then distros like PCLinuxOS become a fantastic option. Another great non-Ubuntu distro (based on Debian) would be Simply Mepis.