The system in Monsters & Magic is like the bastard offspring of OD&D, Dungeon World, Fate and HeroQuest (the Glorantha-inspired RPG, not the boardgame). While I find that interesting, I don't think that everyone will.
Character generation in the game is definitely more fiddly than standard OD&D. A starting character receives a lot of options for their character, giving you a character from the gate that is typically more powerful than your bog standard old school character. In terms of complexity of character, you can get a character that is close to a D&D 3x character. It doesn't take as much work to make a Monsters & Magic character, most of the complexity is baked into the characters and doesn't require the player having to make a lot of choices (like with the feat selection in D&D 3x). You can make a competent character right out of the gate with this game. That is something that I like about it.
One of the "new school" elements in Monsters & Magic are what the game calls traits. Traits are broken down into various types, from character to race to class, and they can provide bonuses to rolls or special abilities for a character. The standard class and racial abilities that a D&D player would be used to are rolled into traits (as are 3x feats, to a degree), in a simplified manner. They work like a mashup of D&D abilities and Fate aspects and HeroQuest traits. These traits are where the HeroQuest influence is most pronounced. While most traits are predetermined, there are places where they can also be player-defined. This means that you can have more than one character in a group inspired by the same archetypes, but at the same time you can still customize characters so they can look and feel different. It is nice to have mechanical support for this sort of thing. I think that character advancement is a hybrid of Dungeon World and standard D&D.
The DIY aspect of old school gaming is well represented in these rules. Throughout the rules, Newton encourages players and GMs to manipulate and add their own touches to the game, and throughout she gives guidelines on converting material from "classic fantasy" games into Monsters & Magic. This is pretty much where the "old school" influences end for this game. Someone expecting OD&D with some extra bells and whistles is going to be disappointed in this game. This is not the first game that I have seen marketed as an old school game that misses that mark.
Don't get me wrong. I am not holding the attempt to be "old school" against the game. It is better to try to make the game that is right for you, and have it fall short to others, than to not try to make that game at all. Each game, whether we like them or not, is a step in the direction of a diverse hobby, a place without cookie cutter games. Sometimes, when trying to do many things, you are unable to put as much effort into all of those things. Despite these flaw, the underlying system is a solid one and makes interesting characters with a strong sense of story behind them, stories that manage to not overwhelm what you want to do in the here and now.
The Monsters & Magic system and GMing advice do a good job of explaining to the reader how the game should be played. There is talk about high level campaigns and how to run them. There are rules for making constructs of all shapes and sizes (similar to the rules that appeared in Legend of Angelerre, this must be something that Sarah likes in her games). The GMing section is probably better than that in many old school retroclones anyway, many of which prefer a "learn it yourself" attitude.
One problem that I had with the game actually comes out of all of the talk throughout the rules about high level play, and the various "scales" of play in the game. Despite the fact that the rules support and talk about high level play, there are only a handful of spells available for the various magic-using classes and those spells only cover up to fourth level characters. My other problem was that there really aren't a lot of monsters in the game, all told. Perhaps games like Swords & Wizardry with its extensive lists of monsters and magic have spoiled me, but it seems like a game with monsters and magic in its name should have more of both in it. Monsters can be fixed by making your own, or converting them from other sources. However, old school fans will find that adapting or creating monsters will be a bit more difficult than they are used to because the monsters are probably closer for many to 3x in terms of complexity than OD&D.
Neither of these hurdles are insurmountable, given work on the part of the GM and players, but when in a market where there is already a lot of fantasy RPGs, these are factors that could cause someone to put down Monsters & Magic and pick up another fantasy game in its place.
Ultimately, you have to decide for yourself if this is the game for you. There are a number of strong points (varied and interesting character creation, narrative game elements in the rules) to the game, but they are balanced by some weak points as well (lack of magic and monsters). There is a good game in here, it just takes a bit of coaxing to bring it out. Hopefully, a couple of years down the line, backed up by a few years of actual play and design experience, Monsters & Magic will get a revision that will address some of these issues. At less than 150 pages, there is space for these things to be added without making the game a monster of its own.