I have a Pentax 67 with 45, 9072.8, 165/2.8 and 300/4 lenses which I obviously only used with film. My experience is that very fine (stunning!) 70x100 cm enlargements are very well possible with scanned images using Velvia and Provia. On the other hand I can see a lot of problems in the scanned images in actual pixels view. Chromatic aberration is pretty bad, which is quite obvious.
One thing I also noticed that images a friend shot with his Hasselblad (same subject, same condition, same time) were better than mine. This may depend on my friend having better technique or the Hasselblad having better lenses.
Now, the Pentax 645 may have better lenses than the Pentax 67, but I wouldn't hold my breath that they are good enough for digital. Just as a reminder:
Michael Reichmann found that his Canon 1Ds? essentially outperformed his Contax 645, mostly because of limitations with his Zeiss lenses. Hasselblad uses a new lens line for their cameras, calculated at Hasselblad and built at Fujinon. At least in the MTF data published by Hasselblad the new HC (Fujinon) lenses are clearly better (MTF-wise) as the old Zeiss designs. Any one can check, MTF data can be downloaded from Hasselblads website.
Michael returned to medium format, but started out with new Rodenstock Digitar lenses, specially designed for use with MFDBs.
A quoute from the Reichmann article: "But after more than a year of shooting both with a 16MP Canon 1Ds MKII and a 22 MP P25 back on the Contax, I came to a couple of realizations. The Canon and Zeiss lenses on their respective cameras were not the equal of their sensors. In other words, the sensors were outperforming the lenses. Even using the best primes on the Canon proved to me that the 1Ds MKII was not being pushed to its limit, and using lesser lenses often disappointed."
The whole article is here: http://www.luminous-landscape.com/reviews/...ital-view.shtml
Only Pentax lenses I owned were the 6x7 format ones - the 55mm and 200mm. The 67ll body was solid but the shutter bounce made me give up in the end. Also, the very slow synch with the huge focal plane shutter was not that wonderful an experience; perhaps if Pentax had made a series of shuttered lenses à la Hasselblad it would have been different; as far as I remember, they only made them in 100mm and 160mm and then stopped one or the other of those. A digital back on that body - I wouldn't hope so.
But when you consider that Mario Testino, Marino Parisotto Vay and Sante D'Orazio have done very well with that camera...