Just think about the performance gap for such a small enhancement. I don't really say AA don't impact, I only say its near useless to go too far if, for example, you are on a 1080p on 22" ou 24".
I don't know about the rest of you all, but I can definitely tell the difference between 4xMSAA and anything higher (8x or 16x). This stands for my current 22" 1080p display and my old 1280x1024 display. 8xMSAA is the least I'll even think about. I don't really use SSAA all that much though, even if it is available in OpenGL, so I can't comment on its use.