I'm using beryl (compiz's fork) without XGL and it's WAY faster then it was with it. XGL was a pretty big memory eater and evertyhing was slow, especially XV and things like scrolling web pages, or with lots of flash content, not to mention the lack of direct rendering. Anyway, everything is smooth 'n sweet with these new drivers now.
This "black-windows" bug is a known ting and is caused by video memory exhaustion, it will be resolved in the future release of the driver.
What I can recommend you to use to avoid this bug is beryl (recent svn build) it has a new rendering method which does not require GLX_EXT_texture_from_pixmap extension, and one more nice thing about that is that it's as fast, or sometimes even faster than texture_from_pixmap mode.
, get yourself a recent copy and enjoy!