Kind of a useless post, being that I don't own one...
I played some with the first version. In my short usage I found it was rather slow to focus (slower than the canon or nikons), and didn't hunt too much.
I felt the weight was good and the ergonomics were excellent. I have used Mamiy's 645s before. I think there lenses are as good or better than any of their peers.
I know two people who own or owned the newer version. One is a wedding and portrait photographer. She sold hers after the shutter curtin failed twice in the first 18 months. She claimed to love the camera, but couldn't trust it.
The other person is a 'hobbiest' who also loves his and keeps adding lenses.
I have had a lot of dealings with MAC (the us dist.) and find them to be very good. I also think the build quality should be good. If you decide on the Mamiya, I would strongly recommend buying from robertwhite.co.uk -- If not the body any accessories. White's prices are hard to beat, even with a weak dollar, you still get a warranty (although you'll have to ship it overseas for warranty work) and they are nice folks to boot.
There is a good thread on APUG (probably many) about the Hassy 645 AF. My personal take on the Hassy is it comes with the Hassy/Zies price without being a hassy/zies camera.