I'm running 10.10 (release candidate) with a Gigabyte HD4650 1GB ATI card. Under the source editor on Ubuntu Tweak I enabled the PPA 'deb http://ppa.launchpad.net/compiz/ppa/ubuntu maverick main'. When the package update installed there was an error involving a file that the install wanted to overwrite that was not allowing itself to be overwritten. The install "finished" and I tried 'compiz --replace', but when the desktop redrew there was no compositing and recurring draw errors. I tried 'metacity --replace' but I was still having difficulty enabling compositing. There was a noted package error icon in the system tray, and when I clicked that, Synaptic launched and said there was a broken package. I fixed the package (compiz-gnome) and installed it, but I still couldn't get compositing. It took two reboots to bring compositing back, but I am fairly certain it's Metacity at this point (I finally got the 'Enable Metacity's Compositing Feature' to take) because when I run the Compiz Config Settings Manager, trying to select any option crashes it.
Do you mean if you checked the 'Enable Metacity's Compositing Feature', you can't enable the Compiz's compositing, right?
And, there's a problem to upgrade Compiz through the PPA, right?