Never had one, but 2 of my mates have, and both ended long term relationships because of it, both ended up with the other girl LOL. I'm still friends with them to this day but it was a bit awkward at first as I was told the story of what happened before meeting the 'new' person, and my wife is still friends with the original girls. I think its best left for people who are not in a relationship and are just having sexual relations with others for nothing more than fun/good times, than if the opportunity arose it would be nothing more than a sexual encounter.
G