I'm running a drm-next kernel with DDX/drm/mesa from git on my r600-based laptop. KMS works well, it gives me nice shineh effects in KDE4 and I'm very happy with it.
I have noticed though that since moving to the 3D-accelerated versions that playing videos in a window takes way more CPU than it used to when I was just on the released versions of all the driver code. It happens with both gl and xv backends to mplayer, and only when in a window not fullscreen.
Is this due to things that are still being worked on in the driver, interrupts springing to mind here, or is it just a consequence of the extra work involved in doing all the fancy compositing effects? The extra usage is coming from X, so I'm hoping it is a driver-incompleteness issue.
It is most likely all the extra crap that's going on...
Compiz drives xorg nuts no matter what driver you use, i.e., it does the same with intel and nvidia drivers, as well as with fglrx.
I know my mythtv box (HD3200 w/ git drivers) has really high CPU usage when playing back videos and also during live TV watching, but that's full screen. In my case it's the X server blocking while waiting for vsync. I'm not saying you're running into the same issue, but maybe it's a possibility.