I did an in-place upgrade of the Ubuntu 10.10 running on my mother's netbook to Ubuntu 11.04. I had it run overnight, and by morning, voila!, I had Natty Narwhal on the computer. The upgrade went by without a hitch; real sweet, considering this was a Wubi installation.
I can't say I'm too fond of the new Unity interface yet, though. I like the simplicity, but I've just gotten far too used to the old way of working. Unity just hides several icons so it was at first confusing to look up applications. That said, I'll keep this on for a month or so and see how this works out for me.
The upgrade offered to replace old staples like OpenOffice.org and Totem with LibreOffice and Banshee. It also put in the new Firefox 4. I took all proffered upgrades as I don't think they'll break any files I have.
Ubuntu One looks much slicker than before. If it works as it should, I'll consider using it again.