After using Windows Vista/Windows 7, and several KDE and Gnome UI interfaces on Fedora and Ubuntu, I have come to the inescapable conclusion that the Mac OS X GUI is the worst of them more than any of them.
The Mac UI is the vi of modern user interfaces. It sucks unless of course you would love vi and all of it's keyboard quirks. Then it is what you should use. I rate it below Gnome 3 and Ubuntu Unity.
Windows 7 and Cinnamon, followed by KDE are at the very top.
That is all.