older games, like FEAR, Doom3, Thief, yeah, I'll run AA/AF maxed, but only because my GPU(s) pick up the slack in regards to performance, and because of the older titles I get really great performance. My P4 chokes the 1950s so friggin hard, though, it ain't right. Newer games, though, I don't even think about it - Crysis, for example, even if I run 800x600 chokes down to 1 FPS with everything set on low if I turn on AA.
As to the whole xFire/SLI bit, it's defi a niche audience. TBH, if I hadn't been able to secure my two 1950s for as cheap as I did, I never would've gotten hold of a second GPU (total spent between the two was about $235). Granted though, most people that do go the dual card route tend to go with the 1337 graphics cards, too, which makes the average dual-setup appear even more expensive.
I think, though, for ATI to be able to offer a card with two GPUs on one PCB, which gives you the median performance between xFire and a single card, and to be able to offer it at a price that is cheaper than two cards - you've got the making of a great product.
Even if nVidia was fisrt to market with something like this, I give them acknowledgement for that.