I've heard nothing but "ati drivers are shit" for like 2 years now, and for most of that its been a non issue, most of the time its Nvidia fans just finding somehting to bitch about tho I'm not saying ATI drivers don't have their problems.
Its been games not developed or tested on ATIs cards and thus they dont run as well as they should, partially the drivers perhaps but certainly not the major factor. Games developed with Nvidia cards in mind will of course have things done the "Nvidia way" and naturally that will conflict with ATI at times. This is my opinion of course
I agree they should both adhere to the standards strictly but that doesn't happen, for either company, they both cut corners to try to get those extra FPS etc.