In FarCry, NFSHP2, UT2K3, GTA3, Q3 and a few others I have the AI setting at high because I'm not able to really tell a difference. In Doom3 I have it set to normal, because at high you start getting the moire patterns on the floor grates (texture filtering optimization -- trilinear first stage, bilinear other stages). The framerate jump is pretty good in both.
As far as IQ, so far as I can tell it's exactly the same as the 4.9 betas -- in some cases it's even better. I also like the fact that the optimizations are "optional", so that I can decide for myself if I want them or not. With the new drivers, that texture stage optimization (trilinear first layer, bilinear other layers) only turns on if you've got AI turned on. The control panel also has settings for temporal AA and the associated framerate threshold.
It seems to me that the 4.9b's visual quality is basically the same as the new drivers with AI on "normal". I honestly can't tell a difference at all between AI off and AI normal, but the framerate change is definitely there.