Having just sold my Mamiya 7 to purchase a M7 I decided to jump in here. There is not much to add to what has already been said especially by Todd. Simon's point that images from a 1.4 'lux or a 2.0 'cron, are different is spot on IMHO. The Mamiya produces wonderful images and they do look astounding on the light table. But if you need faster lenses, or something that travels easier, the M7 will usually win out in practical everyday use, at least for my personal use anyway.
I never did get comfortable with the format. There is something that never appealed to me about the dimensions. But I did make some images I'm very proud of with that camera. But it didn't fit my style any longer and I was relegating it to the drawer and that seemed unjust in some way. It's in good hands today and will continue to make the current owner happy as it fits his style and process.
Larry