I use Mamiya 6 wit all 3 lenses a bit more than a year. A great camera indeed - the collapsing feature is really nice - the camera with 75 mm lens collapsed is really quite compact.
The rangefinder works fine. I find focusing the 150 lens needs some care, but I do get good results most of the time - even close and "wide open".
I have 2 gripes with the camera. The first is metering. On of the reasons I sold my Rolleiflex and got the Mamiya was the AE it offers. But I find it just usable with negative films and too imprecise with slides. Do not get me wrong - the meter is precise - but measures too wide field - more than the coverage of the 50 mm lens and is very sensitive to light from above and easily dominated by it - measuring a "normal landscape" shot were upper part is sky one may easily end up with 1 - 2 stops underexposure. In confined spaces where the light is more even it works fine.
A smaller gripe is - while all the lenses are VERY sharp (one stops down for DOF, not for sharpness) the rendering if the 75 lens is just a bit too hard - in particular the OOF areas at closer focus distance may look "busy" in particular if these have more pronounced structure (lines, spot lights). It is no problem in very most shots, but for portraits I much prefer the 150 lens. the 80 lens for Mamiya 7 seems a bit smoother, but for environmental portraits the new Bessa III would probably be a better solution (my impression based on what I have seen on Flickr).
As much as I like the Mamiya (just check my signature for flickr link) I do keep missing the Rolleiflex. Who knows - maybe one day I will switch back or even get the Bessa III.
The 150 lens being long and slow will (for hand held photography) require ISO 400 film for non-sunny condition. On the other hand fasted shutter of 1/500 may easily push you towards small apertures at full sun, but that is part of the trade. Truth is - the rangefinder would not manage to cope with faster or closer focusing lens with the current design (and the lens would be large).
Still - if one likes square - than Mamiya 6 is the sharpest and at the same time lightest system camera that can be lugged around all day long in a medium sized shoulder bag.
I got mine from a RFF member. It had the film winding mechanism changed in recent years, but still it is not really smooth and one should not use too much force (rewind with moderate speed) as based on what I have read once you break the winding your repair chances are slim as Mamiya does not seem to provide parts for the 6 model anymore (the advantage of the model 7 apart from wider lenses)
