I had a Pentax 645N for a while and the handling was really nice. If I were to get 645 SLR again I would chose Contax if I could afford it (the 80 and 120 lenses are great, 140 is great for portraits) or Pentax 645NII (the 'II' for the matrix metering) if I were after after long lenses (there are great 300 and 400 lenses). The disadvantage of the Pentax is that it does not allow to change film backs mid-roll.

But if you do not need an SLR and fast lenses than do have a look at the Bronica RF 645. I personally find that the 65/4 lens has great 'pop' - just check Flickr.