I got the Mamiya 24mm ULD fisheye during the summer, and used it a little with my 645AFD and 37x37 mm digital back, and also on an EOS 5DII. Apart from the extra angular coverage, I was expecting some image quality improvement over my "very good sample" MC Arsat 30mm fisheye, but I didn't get some improvement - I got a lot
- at the wider apertures and the further off-axis field points. I'm delighted with how sharp and unabberrated it is, even wide open at f4, even towards the corners. My DB's 9 micron pixels certainly don't challenge it at f4 or f5.6 - it laughs that off with a sneer - "9 microns, is that the best you've got?!"...abundant colour aliasing (moire) in the plane of focus proves this. It would perform even better on a 6-7 micron pixel back.
One thing I can't comment on, though, is how it fares closer to the perimeter of a 645 frame; my sensors are not big enough and I haven't tested it on film yet.