I had a play with an MM a few weeks ago in Leica's Mayfair shop, or rather their upstairs gallery area which, ironically, is pretty much all black and white fixtures and fittings - not sure if that is the best place to test a B&W camera?
The MM looks and handles just like an M9 and the files straight out of camera were as I had been led to believe... flat and uninspiring.
Now I'm not a Leica fanboy and very far from any photographic expert, B&W or otherwise, but once opened in Lr4 I find the files extremely easy to make pop. I find subtle adjustments yield amazing results. Much more so than I have experienced with my M9 and Lr4 &/or Silver Efex Pro. I have printed a sample to 20"x30" and the detail is truly amazing. But then the M9 never ceases to amaze me with the detail it provides. ( I had a 35 Lux FLE attached, provided by Leica for me to try, so perhaps that is some of what I'm seeing too).
I am hopefully taking one out onto the streets in a week or so to experience it in the real world rather than a B&W room. We'll see how that goes but I am impressed with what I have seen so far.