I have shot with a Mamiya 6 and a number of other fine medium format rangefinders, and whoever told you that MF rangefinders are not serious cameras/not for serious shooting is simply full of it. Hassies, RB67s and many other waist-level finder SLR cameras can be equipped with eye-level metering finders. I shoot with a number of MF cameras, but my go-to outfit is a Zenza Bronica GS-1 with either a normal (105mm) lens or a wide-angle (45mm) and a pretty accurate eye-level finder. It's not light, but it's manageable (the GS-1 outfit is much lighter than the RB67). I also shoot with a Mamiya 645 (once again, with normal and wide angle lenses). I do not recommend telephotos for medium format work unless you are doing portrait shooting with a tripod. MF telephotos are very heavy and their shallow depth of field is quite unforgiving for small focusing errors. I also like 6x7 much better that 6x6. 645 is fine, but the normal orientation of the camera is portrait, which to me is a minus.
Shooting MF will require some adjustment, especially if you are moving from a smallish APS-C sensor. DOF is inversely proportional to the surface area of the sensor, so manual focusing takes some getting used to. If you can afford a Contax 645AF, that would make things a bit easier. I personally learned MF using a Mamiya 6 rangefinder with a 75mm normal lens.
These are a couple of examples of photos taken with the GS-1 and the 645.
https://flic.kr/p/d5mrVo https://flic.kr/p/a8LeaN
Last edited by ggrpitt; 08-31-2016 at 09:28 AM.