I've only done it once and let's just say I'm looking to do it again. It happened with a couple I knew who came to town, she liked to go to totally nude clubs. We all went, and when my buddy was in the restroom, she asked if I wanted to 3 way with them.... I of course said yes, and we headed back to my place.
I laid on my back, she lay flat ontop of me mounting my cock and he entered her from the top. I obviously didn't have any range of motion, but John (her husband) more than made up for it. His cock isn't quite as big as mine, but suffice it to say it was tight! The feeling of tight pussy stretched over you plus a cock sliding against your cock is amazing and easily the hottest sexual thing I've ever done... HIGHLY RECOMMENDED!