I was just browsing and came across a post which explains what I thought when I first did the swap to ATi, and the main reason I'll be sticking with them unless they really mess up..
When I got my ATI 9800 XT and my HD 5970, I noticed something is "different" about AMD AA. It looks "better" and "sharper" in motion. I can't explain it. I do like their digital vibrance (color saturation) better than Nvidia on my monitor. I had a GTX 280 FTW when I upgraded to the HD 5970. I always picked high quality for bother Nvidia and AMD in the control panel.
It's true. ATi does give a more vibrant image and, to me at least, 2xAA on ATi looks as good as 4x or even 8x did on my 260.