My wife has done the MMF on a couple of occasions (with me and without me) and had a fabulous time of it.
I find it incredibly erotic to watch or participate, something so naughty about this sweet midwestern girl wanting it so bad that she would take two guys at once.
We tried a variety of positions and each of them is such sweet bliss, she loves the full feeling of a DP while I love having her sucking and fucking at the same time.
There was a comment earlier about a MMF not being a guy's fantasy, but there is something about watching the woman you love being so wanton and so uninhibited that makes it really exciting.
We've never gotten beyond the kissing and fondling stage of a FFM, although I did get to watch(along with a number of other people at a party) when she had her first FF experience. We have both talked about how great it would be to have the FFM.