Just tried out the Kororaa Xgl Live CD and it is pretty damn impressive. It's a nice way to try out Xgl and the uber cool effects without risking your existing setup.
There is a file on the desktop which gives you some commands of what you can do. You can rotate desktops using a kind of carousel thing, show all windows by pressing F12, add transparency to windows, get the wibbly wobbly window effect when you drag, etc.
The effects are actually useless and would actually annoy me if I had to use them from day to day. Xgl is impressive though. Opening up glxgears and moving the window around like mad I still managed to get around the same number of fps.
This live CD comes with nVidia (en-vee-dah ) and ATi drivers so it's uber easy to try. Most distros will require you to install these drivers yourself.