Does the 3D HW work well with X11? Is it stable? Does it perform well
(does it support e.g. HW acceleration for Xrender (composite), Xv etc)?
For games it might be fine to (re-)write your UIs in OpenGL and work on
top of a framebuffer, but who wants to do that for e.g. Firefox?
Especially if you could run only one application at the time, and because
the OpenGL driver is a proprietary binary blob, you cannot properly debug
your application crashes.