This might seem lame to some, but I actually decided to replay the original Unreal game several days ago, and I'm about 3/4 of the way through it.
There exists a bug that prevents AA from working under D3D, but I'm not entirely sure this is a definitive bug with the Catalyst drivers. Nonetheless, OGL works fine with AA.
Anyhow, I'm pointing this out because for the first 25% or so, I was "living" without AA under D3D, and finally switched to OGL. At 1600x1200, there still exist those who think AA is not worthwhile, because they believe that jacking up the resolution is just as effective. It's amazing to notice how wrong that really is. Since making the switch from D3D->OGL, the IQ is really a strong testament to the overall quality of the Antialiasing modes available in the R3xx series.
On a sidenote, replaying Unreal using the UT textures really is amazing. Hard to believe that something that old can still look so good.
If you really want to get a good feel for the strengths of ATI's AA, fire up IL2 Sturmovik. Quite frankly, I'm not so sure that I've seen a better illustration of how superior their AA is to the competition.