I have been using 12.04 with gnome shell on my Thinkpad X201. I did an upgrade to 12.10 yesterday and was never able to get the shell properly started; although Unity worked fine. When I tried logging into Gnome shell, I got a desktop and a couple of startup programs w/o proper window decorations, but the top panel never showed up, and I was never able to get to the overview or dash via the keyboard. I was not able to close these startup programs (e.g. Skype); however, I could interact with them.
I had been using many extensions prior to the upgrade, including Maximus, which is a likely reason that the new upgrade system didn't quite work with Gnome shell (misbehaving extensions is my theory). I tried changing PPAs for the shell and extensions and removing shell, but ended up rendering the entire system inoperable. Since I had cloned the system prior to the upgrade, I'm going to give it another try after I restore.
In 12.04 what is the proper way to remove all extensions? and then gnome shell? My sense is that I will have better luck if I install gnome shell and extensions clean once 12.10 boots properly and Unity works.