While in general there is less of a problem with ATI GPUs in OSX than with Linux, there are still more issues with them. In fact my Intel HD 3000 has less issues than my ATI.. as it takes just about everything thrown at it which is a big change for Intel. That said... the Radeon X1600 drivers in Mac OS X are horrid, and have many issues even running native OpenGL apps that work on all other ATi cards fine... so expect problems.
