I've always wondered this myself but we will never find out because people would have to be honest about this taboo act. I do it without shame but on the other hand the only place I'd admit to anally masturbating is a forum such as this where no one knows me.
I've only had 1 orgasm through anal play... and it was awesome. I knew it was coming because I felt my back getting numb... like it was falling asleep and then the next thing I knew, I shot a massive load across the room.
I'm a man and I like masturbating anally. Sometimes I don't even jack off when I'm doing it. Also I watch girl on girl porn while I'm doing it or a girl playing with her self.