Frank Murphy has made an theme selector extension to easy preview and select some great gnome-shell themes done by half-Left
After some testing of this great extension I have some packages for Fedora 15 to make it easier to install and test.
You just have to do this (as root):
yum install http://timlau.fedorapeople.org/files/packages/theme-selector/gnome-shell-extensions-theme-selector-0.9-1.fc15.noarch.rpm --nogpg
then run (as your normal user):
there will link the included themes into ~/.themes.
Then you need to press ALT-F2 and type r to restart the gnome-shell and you are up and running.
Now you can activate the Overview an select the Themes button to show the theme-selector page and start to select new themes.