I love the idea, had the opportunity a couple of times - once with a couple friend, I went first then he fucked his wife, then back to me - she was really sloppy by then but that was his thing, he loved to see the mess.
Another time with the same couple, there were a bunch of us taking turns. I watched a guy pound her for just a minute or two, and then indicated he wanted someone else to take over. I went down on her not realising that he'd shot his load so quickly! She loved it, pushed my head into her pussy but I must admit I backed out fairly quickly and fucked her instead. She did give me one hell of a kiss though!