As for my own experience:
It is not, in any way, worse then windows 7.
The UI takes some time getting used to, but once you are used to it, you like how everything is hidden and that you can go fullscreen on everything. The new "metro" UI does not offer any advantages over the Windows Desktop right now, although this might change once more programs adapt to it. The system also feels quite a bit more responsive, and I have not detected performance differences in games.
I don't regret upgrading, but neither do I feel fabalous about it.
I did get it for 12,50 euros though.