Upload & Sell: On
I own a fair chunk of Mamiya 645 manual focus, medium format glass:
Mamiya-Sekor C 35/3.5 N
Mamiya-Sekor C 45/2.8 N
Mamiya-Sekor C 55/2.8 N
Mamiya-Sekor C 80/1.9
Mamiya-Sekor C 80/2.8 N
Mamiya-Sekor C 150/3.5 N
Mamiya-Sekor C 210/4 N
Mamiya A 300/2.8 APO
Mamiya-Sekor C 500/5.6 LD
Mamiya 2x Tele-Converter N
They're all sharp, including the 45/2.8. The 35/3.5 is rectilinear, not Fisheye. The Mamiya Fisheye is 24mm f/4, and it's quite expensive ($800 to $1200), but has an excellent reputation. The Hasselblad Distagon CF T* 50/4 is a bit sharper than the M645 45/2.8N and 55/2.8N wide open (which is f/4 vs. f/2.8), and they're all about the same (i.e. really good) at f/8. The Mamiyas are smaller and less expensive. The Mamiya 80/2.8N is slightly better than the Hasselblad Planar CF T* 80/2.8, both wide open and at f/8. The Pentacon 6 Biometar MC 80/2.8 and Flektogon 50/4 MC hold their own very well against the Mamiya and Hasselblad. Mamiya offers a comprehensive and affordable lineup, so that's where I went.
Mark Hubsand at http://www.16-9.net claims that the SMC Pentax-FA 645 35mm/3.5 is the best available, and I believe him, but the Mamiya 645 35/3.5 is plenty good enough for me (and I'm picky).
The Mirex T-S adapter is a good solution for me. It has +/- 15mm shift and 10 deg tilt. It's setup so you that when tilt is up or down (the typical landscape use), then shift is side to side, and you can't change this. I generally use tilt with normal and telephoto lenses and shift with wide to normal lenses, so the fixed shift vs. tilt axes really doesn't matter to me.
The adapter rotates 360 deg about the EOS mount, with click-stops every 15 deg, so you can use tilt or shift at just about any orientation. It's well made and solid, but some of the control levers/sliders are kind of small (because they have to be), so it takes a bit of effort to use it - not a problem for me. You can get it with a Hasselblad C or Mamiya 645 mount at the big end, and a M42 or EOS mount at the small end (only M42 for the Hassy mount). I think it's best to get the M645 big end, because you can use it with Hasselblad to M645 and Pentacon 6 to M645 adapters. Unfortunately, you can't get a Pentax 645 to M645 adapter, even thought their respective registers suggest it should be possible (70.9 vs. 63.3mm), because the P645 mount actually extends about 7mm into the Pentax camera, using up all of the available register. I've tried to graft a P645 body mount onto a M645 Extension Tube 1, and it just won't go.
Here's some links for Mamiya 645, Mirex and other MF gear:
Jafa Photography: http://www.jafaphotography.com/medium.html
John Lind: http://johnlind.tripod.com/mamiya/mamiyatext.html
Mamiya UK: http://www.mamiya.co.uk/
B&H list: http://www.bhphotovideo.com/FrameWork/charts/comp_mamiyamed_2.html
P.S. the 300 APO is awesome. I don't use it on the Mirex.