Ubuntu switched their default UI from GNOME 2.x to Unity in 11.04, and after trying it out for a while I decided it felt clunky and awkward, so I decided to try out GNOME 3 from the official unofficial PPA - and man, I'm glad I did. The overall feel is just... more polished than Unity, and even more polished than GNOME 2.x. I like being able to bring up the overlay with a keystroke, then start typing the name of the app I want to launch, or browse the virtual desktop list, or pick an app from the dock. The Alt-Tab switcher has a big icon for every app I have open (like OS X), but if the app has multiple windows, they appear in a kind of submenu and I can navigate with the arrow keys to the one I want. I even love the notification system - when an app on another desktop wants the focus, an unobtrusive pop-up appears at the bottom of the screen; if I miss it, I can clink my mouse-cursor to the bottom right to see all the notifications awaiting me.
It took me a while to make it recognise my preferred keybindings, and I've had to resort to poking through gconf-editor and dconf-editor to make it respect my personal preferences, but overall I really like it, and I'm looking forward to seeing where GNOME 3 goes from here.