Well...he does have a point for some cards..take r500 series for example (such as x1550). Proprietary drivers dropped support for drivers >9.3, radeon opensource drivers get an average of 10fps in older games such as UT2004 or less powerfull games like Touhou 8. radeonhd drivers work, but aren't much faster and are still fairly unstable (15-20fps average, crashing every 10 minutes or so, driver has a long way to go still as the game is still more or less unplayable). Note that this is on a card roughly equivalent to a geforce 7600, and rarely dips below 60fps with the proprietary drivers.
I applaud their efforts and overall my experience with ATI on linux has been great but there IS still a huge problem with them dropping support for some cards. For ones like mine there's only a few options.
1. Stop playing 3d games.
2. work on the radeonhd driver to help support my card. (a LOT is still not implemented, or incompatible with my specific card, I'd love to help but this would be very time consuming)
3. get a new video card. (I'm actually happy with my hardware though..just the drivers lately are the issue)
4. (what I actually do) Patch the proprietary driver for new kernel/xorg-server versions, the changes between a few versions are relatively minor, and easy to debug and track down. It only takes a few hours to get it working on an unsupported kernel version or xorg version, though tbh i haven't tried to get it working >=1.7 yet, I'm running the proprietary driver 9.3 currently on 2.6.31.6-rt19 with xorg-server-1.6.5-r1
I use ATI myself and won't bash them for doing a good thing, but he does have a point, ATI DOES still drop cards from driver support very quickly, and well before many of the cards are adequately supported by the OS driver (which means your system effectively can't update X, mesa, etc, and that latest ubuntu ISO has no option for 3d acceleration for his card without painful downgrades or modifying the driver himself).