So, finally, I got Xgl running on my IBM ThinkPad R50 as well. The R50 has a Radeon Mobility 9000 (M9) based on the R250 chip. Xgl is a nice toy, really usefull are the exposé like window switcher and the over all speed gain when switching windows and desktops. Another hot feature are the ongoing animations while switching apps, in semi-transparent exposé view or while rotating the Desktop cube. A great surpise was, that the GPU stays colder than when running glxgears on normal X. At least, the fan of my ThinkPad remains on slow (and therefore quiet) speed. 
Sorry, but I'm to lazy to take screenshots, there can be found thousands of them already in the net.
But there are drawbacks too. At the moment it doesn't run with KDE (at least not with the whole window manager), thats inacceptable for me, I'm used to KDE and very efficient while doing different tasks in parallel on KDE. Sorry, but 'till now no Gnome Desktop was around that was configured that way to convince me from the opposite. Second issue, it doesn't run with the full resolution of 1400x1050. 
But Xgl is in early developement, although I belief it is farther than just a 0.0.1.
I'm impatient for a Xgl aware KDE version and a better support of my Radeon card. Next step will be to evaluate AIGLX, but that has to wait until I had my final examination on 06 Apr 2006.
Greatest resource in helping me getting Xgl to work was, as always, the great Gentoo community, especially the Gentoo Wiki.