Overall the M series are very nice interchangeable lens RF's. I have an M4-2 coming back from repair tomorrow and look foward to a few projects with it. My one main gripe is that Leica didn't build adjustable diopter into their otherwise well thought out M series film RF cameras.
Same thing with the R series, I would have to get extrnal diopter lenses for the VF eyepiece if shooting through my glasses was too much of a pain. I may get the rubber eyecup though.