I owned a Mamiya RZ67 for a few years. I never had a problem with it at all. I owned the 110mm, and 180mm. I tried the 150mm but preferred the 180mm focal length. Both are great lenses, however.
I highly recommend the Mamiya flash bracket even if you never put a flash on it. It makes the camera handle so much better! As for the AE prism, well I hated it. It makes the all ready heavy RZ heavier. I preferred using the waist level finder anyway.
I used the camera entirely for portraiture. I owned a 6x6 Bronica EC for everything else. I ended up selling the RZ to help fund the purchase of a 4x5 outfit.