Hi,
You don't specify the model you have, but I remember seeing the Mamiya Sekor something-or-other 1000 model for sale in the 1970's / 1980's. I can't say I've ever heard anything good or bad about them, but they always seemed to hang around for a long time in the used equipment shops compared to Pentax Spotmatics, Minolta SRTs, etc.. As you say, Mamiya have a good reputation in MF and so my feeling was that the slow movement of their 35mm SLRs was probably due to their being physically rather large and bulky compared to the opposition. Perhaps this lack of miniaturisation was good from the point of view of longevity and reliability.
Anyway, I look forward to hearing how you get on with the camera.
Best wishes,
Steve