I use 2007 (at work), and 2007 and 2000 (at home). It took me a while to get used to the ribbon - and the new layout - but once you get used to it, I actually prefer the new layout, and now have to top and think about where the equivalent thing is in 2000.

I have heard that 2010 is better than 2007, but that 2013 has some annoying "upgrades" and is slower in some situations. I have 2010 at home and will install that soon, but I think I will hold off on 2013