Not really sure why it would be controversial. The last 4x00 series card released was over 3 years ago in February 2009. Anybody who was running these for OpenGL performance reasons has probably upgraded by now.

As far as driver support goes, the Open-Source driver support is pretty solid. I've been running on a Radeon 4650 GPU on the X.org driver for several months now and really haven't missed anything that would be in Catalyst. Sure, I can't really run Unigine on the driver, but, come on, again, if I was any sort of gamer, I wouldn't be using cards from 3 years ago that lacked OpenGL 4.x support.