I just tried out the very nifty Kororaa Project live CD - a Gentoo live CD with Xgl already built and configured for both KDE and Gnome. It's quite a demo - the screenshots alone don't do it justice.
Your desktop can act like faces on a cube - as you rotate the cube you switch your visible desktop. This works surprisingly well; moving a window from desktop to desktop can be as easy as sliding it from one face of the cube to another.
Also, since everything is an OpenGL surface, you can render video quite nicely and not take a performance hit while all the eye candy is turned on. Windows and UI elements all scale correctly, so you can accurately zoom in and zoom out of an area. Windows also have "bounce" or "wiggle" animations, which makes things feel more plastic and actually gives you useful feedback as you traverse workspaces. The famous OSX F12 key is there too, and allows you to select from all your open windows in one fell swoop.
There are a few drawbacks, both of which happen to be deal breakers for me. First, Xgl doesn't support dual head setups for stretching your desktop across multiple monitors. Second, there does appear to be a performance hit to regular OpenGL applications. Running glxgears in Kororaa yielded about 12,200 frames per second, but in my default SuSE 10 install it runs at about 15,200 FPS. A 20% performance hit ain't good.
Still, the Kororaa live CD is definitely worth a whirl. Since it boots off the CD, Xgl works without configuration and the OS doesn't even touch your hard drive you can try it without risk and see the nifty effects and affects on your own.