Just. Use. Ubuntu. I installed Ubuntu 12.04 on my grandparents’ crappy laptop and it’s working like a charm.
I can't stand to use the thing.
Mostly due to unfamiliarity, I think.
I especially like the new Unity desktop UI. It allows me to launch apps the same way I do on the Mac: by just typing a few characters of the name. Unity still doesn't work as well as Spotlight, but it's a giant step in the right direction.
Start menus are an abomination. Docks are eye candy, suitable for reviving hidden windows, but not much else. On the Mac, I keep my Dock hidden past the right edge of the external monitor.