I just installed XGL with copiz WM on my computer today. All I can say is: wow! It has shadows, transparency, wobble, the desktops are on the surface of a cube.... The Linux desktop is really gaining the eyecandy on the Mac and surpassing Windows. If you wanna see a demo or if you don't run Linux, check out the video: http://www.freedesktop.org/~davidr/xgl-demo1.xvid.avi
Its long, but worth the watch.