I am using Fedora 15 Alpha, which has the best Gnome 3.0 setup I can find and I can say with all honesty and candor that it's in a better shape then Ubuntu's Unity... which is suppose to be out this month.
It's a simplistic interface, but using it comes very naturally after a week or so. The application menu is now finally useful and far far better design then old Gnome-shell versions I used in the past.
The weakest part of the interface so far is that the multiple desktops makes it a bit too hard to find applications running on different desktops. You can easily click on the application icon to bring up the entire application, but often you are instinctively looking at the windows for content, not application. The reason it's slow is because the vertical line of desktop images along the right side auto-hides. I can understand the logic, it seems likely that the designers wanted it to be out of the way to avoid confusing inexperienced users as well as saving space on small displays, but it adds a extra step to selecting windows that slows the workflow down.
I have never been a fan of auto-hide panels, though. If they don't want to make it a configurable option, then it might be a good idea to make heuristic decision on screen size to decide if it is hiding or not.
I expect that people that prefer to use click-file-to-launch approach to starting applications (as opposed to open application, menu to select file) are going to be irritated by inability to embed links to folders and such in their tool bar or their desktop. Gone completely is any notion of a folder-as-a-desktop.
Themes are going to be a problem for a lot of people. GTK2 applications integrate naturally into the desktop, but gtk2 themes are not supported by gtk3 applications, as far as I can tell. I am not sure about this. GTK3 themeing seems a lot better and easier then gtk2 theming, but for now you have to do both.
But it should not be a big deal to port applications from GTK2 to GTK3, I assume... since gtk3 is less of a revolution and more of a evolution of gtk2. I am sure that somebody else can comment better then I.
Things seem to be fast and I am not experiencing any of the same visual glitches and whatnot that I experience with Unity.
I am disappointed that I don't see any Zeitgeist integration for Fedora yet. I don't understand why this is so as it seems that zeitgeist and search are suppose to be core elements to Gnome 3.0.