I was very pleased with my new MM until I put my 75mm Summarit and 90 Elmarit-M on it. My 50 planar was not good wide open either, but tolerable.
The Leica UK line was that it was lens calibration issues and I knew that to be tosh, because the longer lenses were a country mile out - far more than sensor vs film differences would account for and all had been perfect on film. Because my Elmarit -M was non-6 bit they wanted to charge 200 Euros to adjust this one but the rest was free as the camera was new and the lenses recent. I declined the adjustment on the 90 Elmarit and all the kit was returned. Well guess what? They obviously adjusted the body because every lens was now spot on, including the 90 elmarit and the 50 ZM planar that had not gone to Leica.
Now that everything is set up, I find I get perfect focus every time and with no misses. All in all, things are more perfect than my digital SLR and I can be sure of perfect optical performance as long as I focus properly.
The MM is of course largely a M9 and I'm very pleased with the performance of the camera. It runs rings around my 5D III for B&W. The slow writing of files and ridiculous screen grate once in a while though. Lens/body calibration seems to be part of the initial set up really and now that mine is done, it is hugely rewarding to use. I felt a huge sigh of relief when I realised that I genuinely possess a digital M and can use it as I would my film bodies i.e. intuitively, with comfort and no BS I dont need.