I´ve asked I similar question in another tread. I asked what you people thought about the Pentax 645.
It seems no-one answered this part of the question. So, as a total newbie I will venture an opinion. I researched all the possible systems before buying a Pentax 645N. Why this format? Because it is hand-holdable, little bigger than a full-frame digital and very comfortable to use. It does not have the mirror slap issue of the Pentax 67 and other larger cameras. And I can load it in my backpack along with a Pentax digital easily.
The original Pentax 645 is feature poor in comparison and has an interface I don't like. The NII model added MLU which is not really needed. So this is the sweet spot.
As a beginner I love the fact the negatives are imprinted with all the exposure details. A great learning tool. The tripod screw on the portrait side of the body saves me money on buying an L bracket. The built-in handle is comfortable. Really all one needs to add is an extra back, a shutter release cable and a K2 filter.
The Pentax lenses are second to none, much cheaper than the competition.