I'm using Ubuntu 12.04 with Cinnamon. It can be a bit buggy at times,
especially with multiple monitors, but its worlds better than standard
Gnome 3 in terms of usability.
I tried MATE and its ok, pretty much the same old gnome2 (unsuprising
considering its a fork :P). In my opinion it will become less and less
relevant in future as everything will move to Gnome3/GTK 3.
LXDE is nice if your computer is old, I had it running on an old PC at
work and its fast and stable