A little story...
I decided to upgrade an Ubuntu 8.04 install to 9.10.
8.04 to 8.10 went fine. 8.10 to 9.04 broke horribly: the PC seems to have a failing fan which caused it to overheat and die mid-way through the upgrade. Surprising it booted, sort of, but when I got to the login page that died, and Alt-F1 got me to a terminal with a stream of errors stopping me doing a lot.
So I booted from an Ubuntu 9.04 alternative CD, in repair mode, and got myself a command prompt chrooted to the root partition. I manually mounted the boot partition, and ran: apt-get update dpkg --configure -a apt-get update apt-get dist-upgrade .. then exited, rebooted, and got my system back fully upgraded.
I didn't mention, by the way, that the system boots from a software RAID array, which "just worked".
I don't expect things to work out like this when something goes horribly wrong, so it's nice to tell people when they do!
(I used info from here:
http://serverfault.com/questions/8540/recover-from-shutdown-during-ubuntu-di... .. to get it working.)
I'm now midway through 9.04-9.10, just to prove that I haven't been put off!