You might also try gOS, my roommates are both former Mac OS diehards that have switched to it now that Apple has made so many bad decisions. It's based on Ubuntu and runs on a modified Enlightenment desktop environment designed by one of the designers of Mac OS X.
I like it, but I have customized my own Ubuntu install nearly beyond recognition and I prefer to keep what I am used to.
Oh yeah, the next version comes out in April, by the way.