Fantastic piece of kit, I would keep and use it but I'm biased. KEH.com can give you a good hint at the going price. The 645N is a fairly quick camera and indeed it is capable of outstanding results. I think that the 645 (the original older one) is probably one of the few cameras i've ever owned that need no maintenance and are dead simple to use.
It can be near foolproof once you get a few key points down.