WHat i would say about ATi drivers is, same as with Nvidia, some installs will be a bit random and cause some issues due to having something weird open while installing, or some AV that doesn't like the drivers or something going on. I've had issues getting ATi drivers installed on an amd chipset which is kind of stupid but thats life. I've had issues installing drivers and crashes at other times, but that can really happen with any driver at all.
THe main thing to look at is, once driver installing and any real problems have been sorted out and you've got the drivers installed as well as they ever will be, i have zero issues with ATi, but comparitively at the same point with Nvidia in the past 2 years i just will not have a stable system, no matter what happens it simply is not stable. That is where the difference is for me. Mostly its been an Nvidia drivers + Vista 64 bit thing, but thats terrible, its really the system to of gone for from Vista's release, Nvidia always had to go 64bit and always should have had stable drivers for it. ATi managed this for all past cards on vista 64 from release, and from the day of launch of the 2900xt there have been stable drivers for Vista 64.