I just completed a version upgrade from Intrepid to Jaunty. Intrepid was running Gnome and the disk used was a daily build of Jaunty Kubuntu. The entire process was very slick with only 3 decisions to be made by me during the process. This is the first version upgrade I've done since I tried to upgrade 7.10 to 8.10 with disastrous results: it left my system in an unusable state. So far, I have not found any issues though I suspect some may pop up as I work more with the system. In the end I was left with the exact programs that I started with and no new ones with the exception of a few things being different in the system menu and I still have Gnome as the default desktop. I'm very satisfied with the process.
Version Upgrade from Intrepid to Jaunty
Archived thread 1076837 from Arizona Team - US. Markdown source: Arizona_Team_-_US/thread_1076837_Version_Upgrade_from_Intrepid_to_Jaunty.md
It is still an alpha, so be wary that there may be some broken packages.
FYI, it doesn't default to ext4. You have to look for it and specify it.