I have the Pentax 645n and it is delightful. The 645nii adds mirror lockup, unnecessary according to The Luminous Landscape, and I think imprints the lens used in addition to fstop and shutter. The lens imprinting would be nice. While you can use the Pentax 67 lenses with an adapter (which is difficult to find, being made of Unobtainium), according to the instructions some 67 lenses will vignette with it! Strange but true, according to Pentax.
Fully coupled extension tubes in a set of three are out there, as is a focusing extension tube. Also a 2X tele converter.
A really thoughtful touch, common I think to all 645s, is two tripod sockets so that with a quick release plate in each one can switch from horizontal to vertical without significantly altering framing or unbalancing the whole shebang. Genius!
At least in the 645n and nii an ordinary cable release can be used, as the camera gods intended. Praise tha' Lawd! If you really must there is an electric release, shared with at least the PZ-1p. This brings up my perennial question: If the 645n with an electronic shutter can use a proper cable release, why can't EVERY camera offer this option? My Pentax K5 wants an IR remote. Fine, but why not also a cable release? (End of rant!)
My lenses are all manual focus; the introduction of the 645d jacked up prices of all 645 glass and the AF stuff is just too expensive for me. The glass I have 35, 55, 75, 120, and 200 are all excellent; the 120 macro is a marvel, 1 to 1 in one rotation, the 35 (about equivalent to a 20mm on a 35mm format) is essentially distortion free and really sharp.
If you want to do macro, closeups, etc, a right angle finder is available, if you can find one. Worth the search to avoid lying at full length on the dirt to shoot a bug up close.
From what I've read the "Ur" 645, while a worthy machine, is no where as pleasant to use as the 645n. Ken Rockwell mentions this in his "review." BTW, the 220 inserts are easily converted - a small cross head screwdriver and 20 seconds - to 120 or back to 220, a plus with the paucity of 220 film. Oddly the 645 yields 15 exposures on 120, the 645n 16. In my experience frame spacing is excellent.
The 645n and nii offer center weighted, spot, and matrix metering. In my experience the meter is excellent. Have a look at http://www.pentaxforums.com/camerare...ax-645nii.html for a tabular comparison of all three, plus user reviews, some are probably biased.