I've had my Mamiya 645 Pro from new and not had any issues. I chose it over Pentax for the removal backs (which would be less important now). I chose it over Bronica for the instant return mirror, 1/1000th shutter speed and generally less 'clunky' operation. With a winder & AE prism it feels like a camera, the Bronica felt like a collection of components. For 6x6 I chose Bronica because I didn't need it for action - I used my Mamiya for everything often including air shows.
For 645 you need a prism for verticals (fighting with a WLF is an awful experience). It's much easier to shoot 6x6 and crop if you only have a WLF. The Pentax has a second tripod socket on the side making it easy to set up without worrying about L brackets etc.