I have done it several times and it wasn't really all it was cracked up to be, for me. The greatest part was licking two ladies at the same time (one was facesitting me and the other 69'ing). Yum. Dual blowjob (like this vid) was nice but I don't know.. It was less exciting than expected.
But really, I'm more of a romantic/cuddling person. I get more aroused by that even.
I've never done an MFM threesome though, I would like to try it.