Anders,
This is a great system for medium format. There are a few things to be aware of though. The film backs do not have darkslides, so you can't change films mid-roll, unless you 'waste' the unused film and press a button to advance to the end of the roll (not a big deal for me). Also, the lenses do not have shutters in them, so the flash synchronization speed is 1/60th of a second on most lenses, although they have two lenses with shutters in them for high flash speed synching (not a big deal for me, I shoot available light outdoors or with studio strobes). Living with those 'limitations' has not held me back. I would opt for the newer 645N version, as it has numerous improvements that make it very attractive. There is the gorgeous, very bright new viewfinder. It has a dual zone, six segment matrix meter, or you can switch to spot metering or the old average metering setting. No more push buttons like the original 645, it has those big, beautiful dials.
645N offers autofocus with the FA lenses. Manual focus, but with focus confirmation (green light only or green light with audible sound) with the plentiful A series lenses. You have your pick of lenses from 35mm to 600mm. Some fast glass with several lenses having a maximum aperture of f/2.8 and some really nice zooms -- 33-55mm, 45-85mm, 80-160mm, 150-300mm. A stunning 120mm macro - 1:1 magnification without the need for extension tubes or bellows (both of which are available as accessories).
120 or 220 film backs as well as a Polaroid and 70mm bulk film back. Exposure and other data can be optionally imprinted on the film, outside of the actual picture frame - nice and handy.
While there are well used examples out there, this is still a very popular series with enthusiast photographers and so there are a lot of very nice copies out there. Pentax still services the 645N and 645NII bodies. I bought one with a damaged shutter for $100 and had the whole shutter assembly replaced and the rest of the camera clean, lubricated and adjusted back to factory settings for under $300 - a brand new looking and functioning camera for under $400!