The main problem with these big changes is the regressions. The previous Gnome desktop was used for years, and most of the rough edges had been fixed. With the new Gnome desktop, it will take some time to get to that level.
For instance, the screen does not lock anymore when you suspend (https://bugzilla.redhat.com/show_bug.cgi?id=694483). While I am sure this will be fixed, it was working before (on Gnome 2) and is not working anymore (on the other hand, I love that now I have a clock on the locked screen). Each person will find several of these small annoyances ("paper cuts").
I am thinking of going back to KDE for Fedora 15, since KDE's last big change was long ago enough to have most rough edges fixed, and look at Gnome again for Fedora 16. By then, I believe most regressions will have been fixed.