I got reasonably familiar with M&M; read most of it. I’ll go through my background and interests, for context, and then what I got out of it.
Context
I like Burning wheel as a character advocacy/drama game. I like OSR as a game of solving problems in the fiction. Having an OSR game with more focus on social climbing would interesting (as would an OSR game with more focus on stress, fear, combat awareness and such matters, but that is less relevant here).
OSR to me
My interest in OSR is as a style of play. There are different ways of framing this: fiction-driven problem solving, or a wargame in individual scale, or challenge-focused play with a focus on challenges defined in the fiction rather than on the rules level.
The key principles here are: The game master as a neutral referee (since we want to know whether the players are skilled enough to succeed, not whether the game master allows them to succeed) and the fiction always trumps the rules, but this happens within the rules framework present. The first means, the GM is to be an insulated as possible from making decisions that affect how well life goes for the players, instead using random tables, dice, etc. The second means to always consider how to best represent this particular even in the fiction, using the text in the book as benchmarks and framework.
This is the direction from which I write here. This is not a claim about what OSR is in general or really means or at its core or whatever like that.
OSR cred of M&M rules
My impression is that of a reasonable OSR rules framework; one starts with D&D red box, molds it to fit the setting, plays, developes subsystems as needed or as inspired. At some point one takes a snapshot of where the game is at the moment, polished it, and then publishes it as a game. M&M seems to be a result of this kind of process. One difference to much other OSR stuff is that the write-up is quite detailed, including many situations another would have left unwritten.
The game is not explicit about this process, or that fact that good play of the game (in OSR spirit at least; more on this soon) continues the development, maybe changing things, certainly expanding them, maybe removing them, too. The game is a process and a written text is a snapshot of the process in actions. But most other OSR games do not explicitly recognize this, either; maybe the modern ones write “rulings not rules” somewhere.
One specific nice part is how the social conflict rules are integrated in the fiction in a manner unlike Duel of wits or MG/TB extended conflicts. Fine work. I am not going to write anything about whether the rules are good or not, not having tested or throughoutly analyzed them, but for the most part they do not have inherent tension with OSR play, as far as I see, and (as one would expect from Luke et al.) they show a strong and interesting vision of what the world is like.
Game mastering procedures
This includes dire oui, die of fate, scenario/adventure design, etc. Basically: the rules or guidance or procedure given to the game master; how compatible is it with what I get out of OSR play?
I would say it is oblique. The game has inherited some from Burning wheel and the Forge narrativist design tradition. The way of stating them is alien to OSR. They are not really in conflict, but just kinda strange, missing the point, talking about irrelevancies.
There is nothing about neutral refereeing or disclaiming decision making (to use a more fashionable phrase). Failure consequences of magic in particular but also in general are left up to GM fiat; there would have been fine opportunities for random tables there. Die of fate is a crude tool; typically I would just estimate the probability of something being true as best as I can, consult the players to see if they agree, and then roll against that. Here it is always impossible, 1/6, or certain. My typical scale is 1/2, 1/3, 1/6, 1/10, 1/20, 1/100, and complements thereof.
With the lifepaths there is the advice for the game master to build scenarios according to the advancement requirements. In OSR spirit it would be up to the player to come up with those opportunities; better play ambitiously and with purpose if you want to advance, just like in D&D where wandering in the cave is not very good play; you need to go in with a definite purpose and strive for it.
The game is, as far as I understand, missing the actual GM part. Maybe that is filled with random event tables on various time scales, random sea expeditions, random battles or wars; or alternatively timelines for players to mess with; and other tools for neutral refereeing. That would be a nice contribution to OSR urban adventuring toolbox which is so lacking as of now. My hopes are not very high, though.
What is it good for, then?
Personally I read the game as a cruel slice of life one. Take the character, take some opportunity for their life to take a good or a bad turn, and play through to see what happens. Maybe something like the slice of life musings of Eero Tuovinen?
I can also easily see using it for OSR play; just replace the game mastering methodology with standard OSR refereeing and develop relevant tools on demand or as inspired. This is quite tempting; maybe I will find time and players for it some day? Definitely a game to keep mind for urban OSR play in general, too, as it has relevant mechanics to steal or be inspired from.