nVidia and AMD's closed-source drivers have advanced noticeably since Valve released the Steam for Linux client.
I wish it weren't so, and it's nobody's fault but the vendors', but the open source drivers are not clearly better in every way
The advantage of the open source drivers is they are hassle free. They never piss you off by finding new and creative ways to wreck your system and leave you in a bind. For I suspect most linux users, that is the most important factor, not squeezing every last FPS out of your GPU and every last minute out of your battery.
Intel GPU really is the way to go for linux though. No hassle and more or less the sort of performance you would expect from the same chip on other OSes: http://www.phoronix.com/scan.php?page=article&item=intel_snb...