Yes, love it, both FMF and MFM. But you have to be very very careful about the third person if you're in it with a serious partner - you just don't know how some people are going to react seeing you with another person, regardless of what they say. If you sense the least bit of tension or your partner being ill-at-ease, call it off before it goes too far.
Actually, I think my hottest times and best memories of MFM are from my lifeguard parties in college. Both guys were often just good friends with me for a wonderful romp - no real emotions involved. For the occasional FMF, I don't know any guy who would mind seeing his girl with another girls, as log as he can sink his cock into her, too.