I upgraded Ubuntu to Feisty beta yesterday. The upgrade bombed pretty hard. Probably why it's still a beta. I think I managed to salvage things though.
This is another thing I sometimes miss about Gentoo: Gentoo's upgrade messes were always spread out over the course of many months, and then one day you wake up and 2006.1 was "released" and it so happens you already have the equivalent installed, assuming you kept up on updates. In Ubuntu the upgrade mess pretty much hits you all at once.
But Ubuntu makes up for it by the fact that upgrading most of the hundreds of packages on my computer only took 50 minutes of downloading and unpacking. If this was Gentoo I'd still be waiting for it to finish compiling. Even adding in some amount of time to clean up Ubuntu's mess when it bombs (and it does bomb from time to time), it still ends up being way less time than waiting for hundreds of packages to recompile. That's a good thing.
I think it's been six or seven months of Ubuntu at this point, and I'm still pleased. I'm happy with my choice to switch.