I have a few problems with Mutants & Masterminds 2E (M&M2E) at it's core. It's a good game, but it feels just a bit "blah" to me in play. I have issues with the linear d20 system, as well as a bit of a "bored now" feeling toward it's Toughness save in lieu of rolling gobs of damage dice against traditional Hit Points. I heard the Mastermind's Manual would have a few optional rules to help people like me customize M&M2E to what we saw as the exemplary superhero game, if M&M2E wasn't already there. I was mistaken. There are a few HUNDRED optional rules listed herein.
This means the Mastermind's Manual is geared more toward system (crunch) junkies, and setting (fluff) junkies will probably find the book lacking. I am both a system and setting junkie, and bought Freedom City 2nd at the same time as this book to sate both needs ;-).
What you have here is a 160 page tome of pure substance. If you pay for page count the price is terrible, but if you pay for content, the price is peanuts. Nearly every system of M&M2E is given an optional rules and replacement rules treatment, among which are my quest for a bell curved resolution system and hit points/damage system.
But there's so much more. Systems and tweeks that can change M&M2E into a different game. M&M2E at core is designed for closer to Four Color superhero play (like Justice League), but several options presented within the Mastermind's Manual can easily scale it down to gritty Street level play the likes of Punisher, Batman, and Daredevil. Also, some options exist to bring the game into the other direction, scaling the power level up to resemble something like Nobilis (godlings).
Now, I'm not saying you'll use (or even like) all 200 optional rules, because you probably won't. I don't even come close, I only like/use about 10 of them for my version of M&M2E. But having them all there to peruse is a real treat, as it shows exactly what the system can be customized to do, as well as how far the designer will go to help fans houserule the game into their perfect superhero RPG. Every rule seems to have been playtested to iron the kinks out, every question I came up with was clearly answered in the text. While I don't like and won't use many of the rules, it's good to see the breadth presented in an effort to make the book useful to anybody, not just those with a particular taste.
This book also shows how to create Powers from scratch (including breaking down the Powers from the core into their component parts), and handling Powers in different manners than just the core.
The main merit of this book (as in a merit to any group, even those that love core M&M2E with no changes), in my opinion, is the GM tips and tricks. For superpowers that tend to break the plot (you know - invisibility, mind control, esp, incorporeality, mind reading, precognition, etc), utilizing both the "real lives" and the "super lives" of the superhero characters, even different possibilities in the roles of superheroes vs. the society around them.
All this, and I've barely scratched the surface on the Mastermind's Manual, giving you what I would say is a weak selection of the sheer mass of options within. Let me just say that this book turned me around when it came to M&M2E's system. It took me from "blah" to "whoa!" in no time. I can't say as to whether it will do the same to others, but it seems likely to be a good addition to anyone's M&M2E library.