It is all relative of course. Best bet is to pick one up and give it a try.
Chris is correct about the 645 versus 645N / 645NII (the N's). The N's are much more like traditional 35mm SLRs - very natural shift for me. The 645 is not a nightmare, but it does pale when compared with the N's.
I just got back from Ecuador and took both my DSLR + lenses and one Pentax 645N + FA75mm and FA150mm lenses. I wish I had brought more 645 FA lenses, my other 645N body and more film as well as replaced the DSLR + lenses with a point and shoot digital.
The three other people on the trip were vying for time with my 645N kit (at the expense of time with their DSLR). No complaints about weight, size or sound. The experience even persuaded one of the three to go ahead and start buying into Pentax 645N gear!
Originally posted by ChrisPlatt The 645N and 645NII have conventional dials replacing the awful buttons on the 645.
A more informative LCD display replaces the LED display in the finder, too.
But they're still heavy, boxy and rather loud cameras when compared to a classic 35mm SLR.
Chris