Five Nines
I find that something like 99.999% of users take a long time to settle into a new GUI. I'm sure the GNOME 2 faithful will find GNOME 3 hard to live with at first, but if they stick with it they would find it hard to switch back after a year's use.
Most people have used the same system for years and don't use any other system. Those who switch from one system to another regularly find it much easier to adapt to a new system. I do find, however, that those that do use multiple systems don't use as many features as those who stick to the same system all the time.
If, like me, you switch GUIs regularly you probably don't think one system is "better" than another. It's just different. I'm certainly puzzled by those who steadfastly defend their particular favourite window manager as "the best". When I hear OSX users rowing with Windows users or GNOME users with KDE users over who has the better GUI I just don't get it. I've never heard people arguing over who's PVR has the best UI.