I'm working on the Compiz startscript for Ubuntu and have major problems defining xfwm4 as the fallback WM for Xubuntu users. If Compiz is already running, I can't easily replace it with xfwm4 if anything's wrong.

But even apart from that, I think it would be an improvement of the window manager. I use it myself on GNOME, since it's much better than Metacity but setting it up is pretty inconvenient.