I have done it with two different couples: in one, I knew the guy from college and had dated him, and the other was a girlfriend from highschool who I reconnected with in graduate school. It took a while to bring the mutual friendships to a level where the trust was equal in all directions, and we were all pretty sure that no one was going to get hurt. Once the sex started, it was really nice. But you can tell that suspicion and trust issues can come up very easily - you have to be completely open and honest with everyone, or someone is going to get badly hurt, and a marriage might be wrecked.
These are each FMF threesomes - and they are so much fun...