Just upgrade to 11.10 (from 11.04). Waited a couple of months to get a "safer" version.
Unity comes by default - ok.
Since I cannot stand Unity, being an old-time Ubuntu user and having a few applets / shortcuts on the top bar (like cpu,mem monitoring, start a bunch of apps...), I did
apt-get install gnome-session-fallback
and chose Ubuntu Classic in the login menu. Works.
But... the top bar is now almost empty, all my icons disappeared, no more monitoring... Not mentioning the admin menus ...
How can I recover all the icons and menus from my before-upgrade era?
edit
And also, where can I find the system monitoring applets for Gnome 3? (the 11.04 gnome 2 ones do not work anymore)
You have to press alt (or super+alt) to access the right-click menu and to move applets. Otherwise, Gnome Panel 3 is mostly identical, but it has been upgraded to GTK3 and other Gnome 3 technologies. This means that applets also have to be upgraded, and there might be some that haven't been upgraded yet.
You'll probably want to get the indicator-applet-complete package from https://launchpad.net/indicator-applet.