I've got a Mamiya 645 AFD that I bought to try and replace my 35mm system with and honestly, Like what's been said it's too slow and clunky. The AF sucks, it's heavy as hell, 16 shots/roll (typically), and the lenses are slow in comparison. But good lord the scans look amazing.
I use the Mamiya for more conscious, posed, or slower things. If it moves, if it's snapshots, or it's starting to get dark, out comes the F100.