Hi, I had one for many years. Just sold it last week!
I loved the camera. After buying it with the 75mm lens I added the 50mm and sold the 75mm. I think the 50mm is fantastic. The 75mm was also nice but I was really just interested in a wide angle set-up. I never used the 150mm but have heard the same thing Manuel reports about difficulty focusing.
It's very versatile. Relatively light and compact with its collapsible lens, it was my mainstay travel camera. The collapsible lens / internal bellows mechanism works beautifully. Though its body covering is plastic, its build is rugged and substantial.
Like most rangefinders with a leaf shutter, it is free of vibration and nearly silent when pressing the shutter. But the Mamiya 6 is in a class of its own in this regard. I routinely shot hand-held at 1/8 sec, and have had good shots at 1/4 and 1/2 (the 50mm lens is not fast at f4, but the leaf shutter buys you a couple stops).
The controls are straightforward, though I found the exposure compensation dial a bit finicky to use, so generally compensated or bracketed by adjusting the shutter speed after getting the auto-exposure reading, which clearly displays the shutter speed in the viewfinder. I found the internal meter to be accurate and most often used the auto-exposure lock mode which allows a half-depression on the shutter button to lock exposure (e.g., pointing the camera down toward shaded parts of scene to not be thrown off by a bright sky).
I sometimes wished for a larger rangefinder patch but it's adequate. One issue to watch out for, but easy to resolve if it hasn't got to the point of no return, is the film advance lever getting loose and wobbly. There's a screw (reverse-thread as I recall) under the rubber cap on the advance lever that you can tighten with a very thin blade - I suppose there's a special tool for it but I used a paring knife. Anyway, something to be aware of in use. Apparently if it comes all the way off... well, don't let that happen.
All in all, I highly recommend it, especially with the 50mm wide angle lens. It's a joy to use and delivers great results.