What I repeatedly hear is that even with the proprietary drivers, it’s not terribly uncommon for people to still have serious problems, and that not everything can use the acceleration (e.g. from elsewhere in the thread, “VDPAU does not have browser hardware accelerated video decoding yet”).

I’m by no means sure, but the impression I’ve received of AMD GPUs is that all functionality of the GPU is available and functional. And no one ever seems to have trouble getting normal things working.