On my Ati Radeon X1800, when using the radeonhd driver with AccelMethod XAA or EXA and/or with the kernel drm module loaded, the screen shows drawing errors that make the X session partially and in some cases completely unusable.

When using XAA and the kernel DRM module is not loaded, most non-Qt4 apps show drawing errors that make them unusable. Examples: xterm and Firefox cannot scroll properly, OpenOffice.org and many Qt3 apps don't draw most of their controls, only the text labels and graphic icons on them, while the background stays what happened to be in that part of the screen before that window appeared.
Most Qt 4 apps display fine, with only minor problems.

When using EXA and the kernel DRM module is not loaded, all windows stay solid black.

When the kernel DRM module is loaded, the entire screen will become unusable, either solid black or sometimes solid white, either directly after starting the X server or after starting some program that actually uses OpenGL (I used glxgears).

With AccelMethod set to ShadowFB, no drawing errors are visible, but the desktop is rather slow, and both XV and hardware accelerated 3D rendering don't work.

I have already described the problem a bit more in depth, including configuration files and server logs, over at the Gentoo forums here:

http://forums.gentoo.org/viewtopic-t-796671.html

In the above thread, it says I am using driver version 2.5.0, but I have since then upgraded to the newest git version (following the advice on http://wiki.x.org/wiki/radeonhd#head-f79351b4e2b19fad40529ce297ac2d2a1e90354c), and the problem is still there.