I manage to brick my Ubuntu install today by switching the home partition from XFS to ext3. Don't ask why I had my home partition as XFS, it was one of those "it seemed like a good idea at the time" things and I really don't have much of a reason other than that. Why did I switch it back? Well, I like having the ability to shrink a partition and since my Windows one is full of games, the home partition is really the only one left. I was thinking of sticking the Lucid alpha in there to see what happens. Second, I couldn't seem to find an easy-to-install XFS driver for Windows, and sometimes it is handy to be able to access your home folder from Windows - read-only of course, I have some kind of paranoia about Windows and I'm afraid that if I allow the Windows partition to have write-access to my Ubuntu partitions then all hell will break loose.
Then I switched back to Jaunty because my Karmic LiveCD wouldn't boot.
How do I feel now that I'm back to Jaunty? This is probably because it's a brand new install (or maybe ext3 is just that much faster than XFS), but it's speedy. It boots way faster than Karmic did. Things are much more snappy, Firefox starts right away, and things actually respond when I click on them and not 5-10 seconds later. It's nice. Oh, and my webcam works again since I'm back on a 2.6.28 kernel.
What's bad? Old versions of things - I'm back on Firefox 3.0 which is a bit slower than 3.5. I can probably upgrade, but that is something for later. I've also noticed some slight rendering differences on pages that I had open before and after the reinstall, which is interesting. I didn't think that there was that much of a difference. Other than that, I don't really have a problem with going back, and I might just hang out until Lucid comes out to give that a shot!