I just installed elementary OS on a iMac (27-inch, Mid 2011), with AMD Radeon HD 6770M. It generally works well, except one thing:
When I lock the screen super + L, or leave the screen and then come back and the screen is off and locked.
When I unlock everything is unresponsive, or at least very slow.
When I press a button it takes several seconds before I react.
Sometimes I can just go standby, and back and everything is back to normal. Sometimes I have to reboot to solve the problem.