I really like my Mamiya M645 55mm 2.8 N lens. It is small, light and sharp. I tend to use it as a "normal" lens, in the same way I use a 35mm as a "normal" lens with my 35mm cameras.
The field of view is similar to the field of view I see normally, without a camera, using both eyes. It is wide enough to include context, while being "normal" enough to use for people (although not well suited to close ups).
As far as I can tell, all the Mamiya 645 lenses are reliable. I haven't observed problems with flare, and the Mamiya lens hood designed for use with it seems to be a good compromise between its ability to provide shade, and small size and weight.
As indicated above, it will fit on any manual focus Mamiya 645 body. Apparently, you can even use it on the AF bodies, although with limited metering functionality and stop down viewing.
My most recent gallery upload was shot using this lens:
(there was a url link here which no longer exists)
Hope this helps.
Matt