Like many others, I couldn't endure the wait and moved my main desktop from Fedora 11 to 12 a few days ago, in my case using the Beta RC2, so I could take advantage of the install DVD and perform a clean install (BTW, 'ext4migrate' as an Anaconda argument was helpful with moving my /home from ext3 to ext4).
The move to the Beta was the perfect opportunity to get a glimpse of the much hyped GNOME Shell preview, you know, is like going to a road you pass next to a horrible accident but you can't avoid looking at it, you are curious to see what is about. The same with GNOME Shell, from seeing screenshots and reading about it I had a good idea about what to expect and was expecting to dislike it, but I hat to see with my own eyes, try it for a couple of days, leave the first impression settle down and only then write about.
The very first impression is created by the color scheme, which feel originated from someone with a serious Vista envy. Hell, if I wanted a Vista wannabe I would have used ...KDE 4. No, thanks.
Then I tried to do some work done and see hot it feel. As an advanced users, I don't use a single application at a time, I have at least 3-4 windows open at any time and switching from one window to another is a very common operation. Not with GNOME Shell, which make this a pain: you get the desktop zooming and jumping all the time, which is very tiring. And needing additional mouse clicks.
It was so painful that I felt the urge to make it go away as soon as possible. Good luck with that! It was impossible to find a way to kill it (later when I built the courage to try it again I found an ugly and huge 'control panel' and logout buttons are available by clicking on your own name, which I find unintuitive) so I fired a terminal, manually started gnome-effect and said goodbye.
Looking at it my best hope is there is enough time until GNOME 3.0 for this experiment to go the way of the dodo and never make as a default, It may be a pretty toy for some and may have an use case for lightweight usage of your computer (isn't Moblin, also included in the Beta, better suited for the task?) but is far from something I could use for real work.