i've been able to handle the contax 645, hasselblad h1 and h2, mamiya 645af, and pentax 645. i'm keeping an eye out for the pentax 645n/ii, the bronicas, and the older mamiyas.
contax 645 - well balanced with 80/2. very solid feeling, heavy, reminds me of the late 90s. viewfinder is noticeably smaller and dimmer than the hasselblad and mamiya, with a warm color cast. a little harder to focus than the hasselblad and mamiya. the shutter sounds very nice. great controls.
hasselblad h1 and h2 - very well balanced with 80/2.8, best of the bunch. solid feel, but plasticky. design reminds me of the common dolphin. viewfinder is large and bright, with a green color cast. odd shutter sound, like something is rattling around. it's supposed to be like that. didn't really like the interface.
mamiya 645af - back heavy with 80/2.8. solid feel, but the contax is nicer, of course. prism looks like it has a heavy brow ridge and sloping forehead, just like a neandertal! viewfinder is a little bit smaller than the hasselblad, but just as bright, with a warm color cast.
pentax 645 - well balanced with 75/2.5. solid feel, even though it's comparatively light. dim viewfinder, about the same size as the contax, with a warm color cast. didn't like the interface.