I have just upgraded to Feisty Fawn (i.e. Ubuntu 7.04) and it all went so smoothly there is nothing to write home about. I’m almost disappointed 
I took an hour of letting the upgrade tool do its work.
The only small annoyance that I have noticed so far is that it changes menu.lst the way it always does when you do a kernel upgrade. It takes Windows off the list, which is quite annoying for a dual-booter like me. More in another post.
