I'd venture a guess that very few will be lucky to own more than 6x7cm fisheye :-)
Originally Posted by NJS
The focal length difference is interesting: The Mamiya RB67 is actually an 8x8cm camera, and all the RB lenses cover (at least) this format. This is probably why they went with a slightly longer focal length (37mm) for the 180-degree fisheye in this format, possibly allowing a bit of corner clipping if you're one of the few people shooting 6x8cm in an RB67.
I'd love to participate in a comparative test - who has a Pentax 35mm? And unfortunately I'm in South Africa :-(