For once I have to agree with IPAndrews wholeheartedly.
OS X interface makes me scream bloody murder most of the time. Not to mention that the Mac keyboard is slightly different from the standard PC one. I don't know how it is with the US/UK keyboard layout, but Fin/Swe layout is a pain in the ass because several keys are replaced by key combinations. It outrages me when I have to google for those key combinations because I always forget them.
As much as I want to like my MacBook, it usually just makes me angry.
As far as user interface goes, I prefer Windows, Gnome and KDE any time over OS X. The only interface I can think of that is much harder to use is Enlightenment. It is supposed to be lean, mean and powerful interface. Yet I couldn't figure out how to use it well. They are like exact opposites, yet the end result is the same.