I got a 645Nii a couple of years ago. The pics have a silky smooth texture to them:
User Photo Gallery - Macro
Using a larger camera (and having but 16 shots per roll) slows you down and you tend to think more about what you shoot.
Although the camera is handholdable I always use a heavy tripod since I'm working slowly anyway.
I must admit that compared to the K10D the 645 hasn't seen much use.