Awesome work on the poll Piper! :D
On the occasions I've been lucky enough to do it, I've always enjoyed it. And if the opportunity presents itself, I'd never turn it down (:p), but I think the fantasy is probably better than the reality. You mention the visual side of it in one of your questions, and when it comes to what turns me on porn-wise, anal is mighty high up on my list. :)
As to why it's so appealing, I think there's a definite aspect of power/dominance that comes with it on some level. The idea that a woman would be willing to let you do it is just such a huge turn on. For me, anyway. ;)