(As a Hy6-Sinar54r user (just for a few weeks) I would lile to throw in some points about this system:
Since the Hy6 is a 6x6-system you can use a rotating back and switch from vertical to horicontal very fast and convenient. If Waist level finder might be of interest this makes the camera very well for wasit level finder use. If you shoot the Hy6 with other finders the rotating back is still great because you will not need any vertical grips and still have a very good grip of the camera.
On the other side the lenses for 6x6 are somwhat bigger and heavier compared to 645 lenses.
I would think the higher weight is not only needed for the bigger format and the leaf-shutter-also feel mecanically very nice and solid.
I dont have enough experience to comment on the optical quality-but what I see so far from the 40 Schneider, the 80 Xenotar and the 150Zeiss looks pretty good to me.
The prices for new lenses are high but you can get used samples for reaonable prices (for example I paid 470 € for the 150) -still higher than used mamiya lenses.
Personally I find the camera and lenses at least as important as the back.
The back will maybe be replaced when sensors are further developped, but if you have good lenses and a camera with a good user interface you might keep it for a longer time (eventually
)
So here are the factors I would want to add to the ones which were allready mentioned:
-evaluate if WLF might be something you want to use sometimes
-would a rotating back be helpfull if you switch often between vertical and horicontal
-weight and size of the systems (do you want to use it as only system, or together with a smaller SLR?, do you want more DSLR feel or more traditional medium format feel-personally I am happy with a bigger MF-system since I also use a Nikon and a M8 system-this might be different if it was my only system)
-lenses: The range of Hy6 lenses is somewhat limited but than again there is some really great glass included.
Cheers, Tom