I've been using Ubuntu Oneiric for 48 hours now. It will be released in a few days time so I expect my experience to be representative. Verdict: Avoid for as long as possible.
I encountered a heck of a lot of bugs mostly in the shape of programs crashing. It was a bug in natty that pushed me into upgrading early and fortunately that is fixed, but oneiric comes with a lot of different new ones. Yes I reported almost all of them, with about half being known issues affecting others, some for many months. (Do any bugs ever get fixed in Ubuntu?)
There is now a sea change going through the industry for the presentation and interaction of the "user experience". Never before have we had so many different form factors from tiny little phone screens all the way through large multi-monitor setups, using one or more fingers, tablets, mice etc. Microsoft, Apple, Gnome and Canonical are all trying to figure out how to address that. You the unlucky user are the one being experimented on.
If you upgrade to oneiric then you have no choice but to be part of these experiments as the prior desktop environment is not available. I kept cycling through Unity, Gnome Shell and the Gnome Panel fallback trying to find one that works for me. (Yes I know about KDE and XFCE, no I have no intention of using them.)
Gnome Panel has far too many problems. Huge titlebars, clocks cunningly using a dark grey font on a black background so you can't read them and many other issues.
I really don't like Unity. It comes bundled with the menubar at the top of one of your screens (all justified by Fitts' Law) which is unusable if you use focus follows mouse. Lots of niggly other annoyances.
So I am resigned to using Gnome Shell as the least worst. I really wish Canonical had put their effort into it instead of their childish behaviour and NIH attitudes.
Fortunately there are various extensions you can get that make it more usable. Sadly most haven't been adapted to Gnome 3.2 yet. But I've learned that I really need a taskbar because it uses text to distinguish programs instead of little thumbnail windows or icons that all look the same.
No matter which of these environments you choose you lose the applets you used to rely on. I used to have system monitors, cpu frequency scaling, disk mounters, quick launchers, battery time remaining and numerous other frequently used tools across the top bar of my screen. Now you pretty much have nothing. The vendor experiments meant that the plumbing underneath had to be changed significantly hence all those things that used to work no longer being present.
Now I know how fruit flies feel.