I don't have the Mamiya 645 but rather the Pentax 645n. Since the cameras share the same format, my experience may be useful. I have the 55 mm wide angle, and the 35 mm wide angle. The Pentax 35 mm is a fabulous lens, very little distortion, sharp, all the good things. Mamiya makes really good stuff, so I'd imagine their 35 mm would be really good.
I bought the 55 mm first, and later the 35 mm, for which I paid what was for me a really scary price. It turned out to be NOS, even had the warranty card! I find I use the 35 mm much more that the 55 mm; It would be fun to find the Pentax 45 mm in such a case I might sell my 55 mm which to my taste isn't wide enough.
Of course with the 35 mm keeping things level and plumb is pretty important, especially if buildings, etc, are included.
Here's a post from photo.net speaking to 645 lens approximations to 35 mm format lenses:
Jean-Louis Llech , Jan 04, 2008; 02:19 p.m.
A comparison between actual dimension of 645 film format (41.5x56mm) and 24x36 gives a 1.61 factor.
For Mamiya 645 lenses it gives : 35=22mm, 45=28mm, 55=34mm, 65=40mm, 80=50mm, 120=74mm, 150=93mm, 210=130mm and 300=186mm
So its obvious why a 35 mm on 645 film is a pretty impressive lens. And probably pretty expensive, as was my Pentax 35 mm.
BTW, I envy you Mamiya 645 users that 80 mm f1.9. Must be fun using that thin depth of field, not to mention the "available darkness" possibilities.