Xgl is pretty freaking nifty. My friend got it all nice on his computer (my graphics card was having some issues with my gentoo install and xgl so I don't have it myself) and it was looking sweet. But there are so many cool features it is hard to describe it with words. I reccomend looking on youtube for videos of xgl in action.
My card has a really weird glitch I couldn't seem to fix. It makes all windows in black and everything else in white. Couldn't see anything. I even saw on the wiki of a fix, but couldn't get it to work. It worked windowed once, but it was slow and glitchy. Doesn't bother me though.
I would love to play with XGL, however, I am too broke to spring for anything remotely decent in the video card arena so i will just have to wait, listening to others who have used it with success.
If I have to compile ONE more thing in FreeBSD I am going to freaking scream. That's why I'm sitting here on my dull little eCS machine and not using XGL.