Not too long ago my fiance and I tried it with friends of ours. A couple we've known for about five years. After a couple of drinks one night, we told them about the handful of times where I watched my girl fuck other dudes. Part of me figured they'd be judgmental or even critical about it, but instead we were just enthusiastically asked, "Oh, so ya guys swing?"
It wasn't too long after that the four of us had traded partners and were having some fun in the living room. The wife friend I was with was really cute and even though she had this awful, huge tattoo on her hip, I found her really fucking sexy. Still, and this might sound crazy, I was way into watching my girl get fucked instead of focusing my full attention on the wife. In a weird way, it was almost like I was fucking her while pretending she's my girl...AS I'm watching my girl get banged maybe 6 or 7 feet in front of me. Hopefully the wife didn't notice. She didn't seem to.
Afterwards everything felt a bit awkward. These were friends that we'd visit maybe every few months. In one night we went from a cool friendship to telling them intimate details of our sex life to ultimately making them part of it. But afterwards we all had another glass of wine or two and my girl and I staid the night. In the morning we woke up, the wife made us all a great breakfast and we carried on as if nothing had changed. It was very cool, but bizarre at the same time.
My girl and I never discussed doing it again. We both had fun and we plan to visit the couple again sometime over the summer, but we didn't really talk about it after the car ride home. I can tell that it's not something that she'd want to make a regular thing and I'm kind of right there with her.