I attended an event and had a chance to play with the Phase One/Mamiya 645AFD series camera. If you are a studio person and need the ability to make huge enlargements then perhaps it is worth considering. I spoke with several commercial guys and they are pleased with this system. These guys are shooting still life stuff with 45 megapixel backs. Me, I just don't see it, but it is their bread and butter so who am I to argue.
I am planning to rent a system for an upcoming shoot and see how it goes. I know my magazine art directors love the bigger 5DMk2 files so if that means they choose me more often then it is a factor. With Uncle Bob running around with Mk3's and D3's the new differentiator may well be a MF digital camera. But with iso topping out around 800 it is certainly not for most wedding or low light folks, but if you do posed work where you can control your light and need this kind of file size it is interesting.
But they have some serious hurdles to overcome. Price point needs to come down a bit more since you are looking at a new line of glass as well as the camera system so it gets pricey real quick. And I was not wild about the af points and a few other things that I am used to with the way I shoot. But in a studio setting the files are pretty spectacular! I was very impressed with what I saw on the screen and the test prints they had.
Remember the days of the $13,000 1.3 megapixel camera? We are getting pretty spoiled with high frame rates and clean iso 1600. I mean really, how many people routinely shot iso 800 much less 16/3200 until 5 years ago.