I have installed the 12.10 beta (with all updates) on my iMac Early 2008. One annoying problem I'm having is that, when I log out of my account and go back to the login screen (lightdm, per default), the screen stays on: it never goes blank. This is not good for my display I guess!
The screensaver (set to just a blank screen) works just fine when I'm logged in. The problem only appears at the login screen.