Tom, I use a Hasselblad 30mm with a Phase One P25 back and it works fine as a fisheye, and in emergencies you can indeed straighten out the image and use the lens as an ultra-wide rectilinear.
The image circle of the P25 is about 61mm so it effectively edits out the very lowest quality from the corners of the 30mm, if you're reproducing the straightened out shot in a brochure at A5 or smaller it's fine. At A4 in my opinion it's at the limits, and at anything above A4 the lack of resolution with a straightened out shot starts to show.
I disagree that it's a particularly rare lens, I've got an old C series 30mm, so they've been around for years. And Hasselblad always did a good job of marketing and promoting the 30mm, which means you'll see them turn up on Ebay every few weeks.
Thanks everybody for helping,
I tested the new 40mm which was very good, I hoped a less expensive older 40mm would probably be nearly as good. Hmm, For the few rare interiors shots, the new 40mm is simple too expensive. The 38mm of course sounds really good, but of course you're limited the the SWC body without movements.
If Hasselbad see a future in their V-System and bring recent large sensors in their new designed V back, maybe they also think about mounting their HC 35mm in a V-series classic mount too, I suppose the image circle isn't that smaller. But will they calculate and design a completely new lens...probably not.
I never met someone with the 30mm, so, I thought it would be a rare lens. V lenses prices seem to fall and fall, maybe i pick up one.