AMD has been completely rewriting the drivers as free software. The 2d performance of the in-kernel drivers is now better than the proprietary driver for southern islands(HD7000). The 3d performance of the free driver is at about 50-70% of the proprietary for SI with linux 3.13, according to phoronix. The latest cards C.Islands use the same drivers as SI so the performance should be similar.
> and it likely still is.
Actually everything just works
http://www.phoronix.com/scan.php?page=article&item=amd_radeo...