My GF and I have done this more times than we can count. Each one was great! Ours were MFMs. We started when we went to a swing club in New York City. We had several MFMFs with other couples, and then we had some MFMs. We found that swinging really strengthened our relationship, eliminating any of the normal jealousy that goes on.
We have been swinging for eight years, and usually have a small bi swing party every month with another couple and four single guys. She has found that what she likes best is to have three guys with her, one sucking each nipple and another sucking her clit.
We love to watch, and hear, each other be sexually pleasured by other people. While the swinging is fun and gives us a lot of excitement and pleasure, we really treasure making love together. Usually after a play session, we will either finish it off with sex together or have a session ourselves when we get home.