OP, GRHazelton's summary is well worth reading. I think he has summed up the 645N in a very readable way

I think the only point I would add is that the AF can be a little slow when attempting to take pics of kids between say toddling age and when they will at least stop for a second and listen to you.This is the perpetual and unpredictable motion age My experience of snapping a very lively granddaughter is limited but the AF often failed to keep up. The 645N will take great negs in this kind of shooting but just be prepared for a few focus failures in the way that might not happen with a F5 and AF lens