There's not a whole lot of information on the web about real world results using medium format lenses on 35mm cameras. Wanting to see if I could use one of my Nikon film SLR's as a back-up to the Pentax 645NII, I set out to do some tests of my own.
First thing I needed was an adapter that wouldn't be too expensive, that was machined to tight tolerances, and would be durable for use in the field. There are some major differences in adapter prices out there, from cheap to way too expensive. I ended up going with the Fotodiox Pro PT645-NIK.
My first thought was to use the Nikon F and an external meter since the adapter does not have the ability to operator the aperture. But I discovered that the Nikon F5 has the ability to meter without aperture coupling. You need to manually focus the Pentax lens and set the desired f-stop. The Nikon F5's light meter will set the proper shutter speed and fire normally. The only downside to this, is that you are focusing at the working aperture. This means that the viewfinder will be pretty dim after f/8. You could get around this by focusing first, then setting the f-stop, but that may get tedious after a series of shots.
Another feature that helps, and is fully operational with the Fotodiox, is the focus indicator light. When used to confirm focus using the correct technique, this process can really produce repeatably sharp shots.
This system is great as a backup when bringing two medium format rigs is out of the question. It also has an added benefit of providing multiple angles of view with one lens. Though I would not run out and get 645 glass to shoot on a 35mm body exclusively. The added conveniences of 35mm SLR technology is why so many people have bought into the systems in the first place. Small weight, image stabilization, wide apertures, are all but non-existent in the medium format world. Even with Pentax's smallest 645 prime lens, the 75mm, the lens and adapter combo is the size of a small zoom on the Nikon F5. Again, not a bad thing if a Nikon body is your backup, but a bag full of 645 lenses while shooting 35mm is somewhat of a waste. If you are going to sacrifice the creature comforts of Nikon's (or Canon's) glass line up, you mine as well be capturing your image on a large sheet of film.