I have been a part of threesome, more than once, and with two different couples, though not at the same time. I find it very interesting though if not all parties are completely onboard with the idea it can get extremely awkward really quickly.
Both of these threesomes were after my divorce. What caused the end of the first one was the fact that the woman broke up with the guy then turned around and went on a date with my ex-husband and bagged me behind my back. WOW - Lots of drama from that one.
The second one was for a one night stand as a birthday 'present' from his wife. Needless to say that was an interesting one.
I would do it again, although in every case I ask that all parties have protection so no one comes up pregnant and there's no question about who's the daddy.