After the upgrate to Ubuntu 11.04 I was directly on Unity. Having a dock on the left side is nice, but unfortunately, it has various bugs which makes it sometimes annoying. Also the menu on top like Mac Os X is not a bad idea, but it breaks many apps (for example Picasa). Then there is the scrollbar insanity, it's almost impossible to click on to scroll, and again breaks in some apps (for example Eclipse). Fortunately one can disable all of that and go back to the old Gnome 2.
Then I decided to try Gnome 3 the past 2 weeks. I was curious that Linus found it so bad. At first it looked nice, probably because it's different. But very quickly, it becomes annoying: the full screen changed just to switch window with the mouse is a stupid idea. And then installing Gnome 3 in Ubuntu is not such a great idea: Rhythmbox does not work well anymore with it, all my "file open" mapping was gone and replaced with garbage: why does it propose GIMP to open a PDF is beyond me.
Gnome 2 + a nice standard dock + widgets (or the nice unity indicators) + an indexer would have been perfect. Now I am back to KDE 4 which provides all of that, even if it feels a bit bloated.