In this particular case the upgrade was interrupted. I have around 250MB on my / partition now. But as I prefer to upgrade from text console, I caught all "out of space" messages and fixed it trivially (made more room, repeat the command). That's the only problem I ever had with a Debian upgrade. I typically use testing.

Now Ubuntu upgrades are hidden behind a GUI, and you are told not to do it manually. It's not uncommon that they fail to cover all difficult cases. Either way, I have Ubuntu on my mother's machine and I've become scared of making upgrades. A few times I had to drop to IRC to ask for specific instructions because I couldn't figure it on my own.

Ubuntu =/= Debian. I have several servers that have been upgraded from Woody up to Squeeze (that's 5 consecutive dist-upgrades in 9 years) and it's less problematic with each release. The only serious difficulties were switching from xf86 to xorg, then apache to apache2. Switching kernels (2.4 -> 2.6) or libc was quite painless.

Since Etch (Etch to Lenny to Squeeze) except for a couple of minor glitches going with the defaults "just works".

From what I've seen Ubuntu often fails upgrades because of the GUI (at some point during upgrade one or other part of the GUI fails/restart and kills the upgrade process itself). Simple advice: dist-upgrade in a text console (not a gnome-terminal!), and it should work fine.