I have two Ubuntu 7.10 systems that I tried upgrading to 8.04. The first (x86_64) encountered a bunch of problems during the Clean-Up phase, causing a whole slew of package system complaints on reboot, while the second (x86) failed during the Installing/Upgrading Packages stage, leaving my system in an “unusable state” and leaving me unable to start Gnome.
More details are on Launchpad: the first system were all semi-automatically reported tickets using the new error reporting system (which sets the tickets to be private), and one ticket for the second system.
Last time I tried upgrading was from 7.04 to 7.10 on my Dell Latitude D600, which also failed and brought my system into an “unusable state.”
Follow me on Twitter for stuff far more interesting than what I blog.