(I got a copy of this book through NetGalley, in exchange for an honest review.)
I found this book to be a very entertaining and refreshing read, focused on the characters' adventures, and likely something I'd have thoroughly enjoyed in middle school—well, I did enjoy it now, after all. In my opinion, it definitely reaches its aim of providing younger readers with likeable characters, all with their specific traits. Penny, the mad scientist, whose power works in spurts of genius, and whose voice throughout the story I quickly grew fond of. Claire, cute but also resourceful, especially when it comes to using connections to the team's advantage. Ray, who simply was, well, classy, and whose take on circumstances was definitely contagious. At the same time, I could tell they were kids, reacting like kids getting powers, and wanting to have fun with those before it was time to get all serious and act like grown-ups.
Paradoxically, the fact they were kids made the community's reaction to their presence somewhat more believable: with heroes and villains alike used to fighting each other with magic and "adult" powers, throwing in kid-like actions in the middle can be unsettling, and difficult to predict. The Inscrutable Machine did exactly that. As for the heroes/villain community itself, your mileage may vary on the "believability scale". Personally, I found it mostly made sense in its own way: with both sides having super powers, at some point you have to reach some kind of agreement to avoid really wreaking havoc with no one the winner at the end. (That, and the alien invasion in the past, which seemed to have forced them to cooperate.)
Another likeable side of the story was how it really focused on its premise, a.k.a. teenagers accidentally labelled as supervillains, then deciding to play the part until they could "see the errors of their ways and become heroes". I've read too many books that start off with something interesting like that, then veer off toward teenage drama/love stories, with those not being the reason why I picked them in the first place. There is a touch of romantic interest here, but it's subtle, and played in a somewhat comical way, so it agreed with me in the end.
The few qualms I had about this novel were mostly related to the adults' reactions. While some of them were understandable (Brian being too focused on his own science, too much of an air-head when it comes to relationships; the Minx as a playful, secret-loving woman who found fun in what her kid was going through; Lucyfar, who openly plays on her ambiguity as a villain who occasionally does good, so she's left alone...), I thought the Audit, on the other hand, was too oblivious to what was happening. It would've worked if she had had a different personality, but I wondered why she, among all the others, didn't catch that something was going the wrong way? Perhaps this could have been played a little differently. I also wondered about Ray's family: he seemed frightened of their reaction, and so I wish we had had more of a glimpse into that, more explanations. (Unless this book's meant to have a sequel, in which case this question might get an answer later, but I don't know about that yet.)
I'd give this story a 3.5 stars, because of those problems that kept nagging at me. However, I'm still rounding it to a 4. I had fun, plain and simple; I think younger readers would have fun, too; and so I think this has to factor in.