Testing rv280 with Linux 184.108.40.206 and F12 userspace
The bottom line: a stock 220.127.116.11 kernel with the Fedora 12 userspace can run compiz. However, the colours all go horribly wrong if you try to rotate the cube. (gnome-terminals can turn completely black inside, or the entire screen can gain a red tint until the cube face settles back again.)
Trying to look at xine-ui's on-screen display while playing a video file freezes the X server but doesn't seem to lock up the card.
Under 18.104.22.168, the VGA-0 connection was configured to 1400x1050 by KMS. However, 22.214.171.124 resorts to 800x600 instead. In both cases, I could return the display to its native 1680x1050 via xrandr.
I tried Fedora 12 with a radeon mobility 9000 (RV250) card. I only tried the compositing, though. In Fedora 12 and Mandriva 2010, I can use either type but with other distros, it would either not work at all or it would crash on a certain one.
I didn't even bother to try the cube or other effects since the support was so poor. 3D with the OSS driver is abysmal, I think.
If you look at the Radeon Xorg page with the various features supported, a lot of it is 'not working', 'unknown' or 'in progress' with few mentions of 'working.'
3D on r1xx and r2xx may have regressed in some cases since the transition to KMS support as fewer people test the older hardware. If you are seeing regressions please report bugs: https://bugs.freedesktop.org
Apart from that, 3D support on r1xx and r2xx is pretty much stable and feature complete.