I have the 645n with the 35, 55, 75. 120, and 200mm lenses. It is marvelous. The lenses are sharp and they all have that proper "feel". I've gotten consistent, excellent exposures with the metering system, and the TTL flash mates properly with my Pentax AF400T flash system.
As others have said the viewfinder is excellent. The focus confirmation is usefull, but not essential. Two tripod sockets, a stroke of genius. And the camera uses either an electronic cable release, or the old fashioned sort the way the photo gods intended. BTW, the film inserts are easily converted from 220 to 120 with a small screwdriver and care. The 220 inserts are generally cheaper. Go for a 645n, you won't regret it.