It sounds exciting, and in many places The Enemy sort of lives up to its premise. There's action, kid-killing-grownups in large battles sort of action. And while it may not be on the scale of Lord of the Rings, sure it's action-packed. Inexplicably, the book dragged for many sections, dedicating entire chapters to kids sitting there planning. And while kids arguing is always entertaining, it hardly makes for a top-notch novel. Strange things happen and aren't explained — such as why the grown-ups are growing some sort of intelligence by the end of the novel.
The book really falls short when it comes to its characters. There is no main character, though Maxie would be the closest we get. The point of view frequently shifts. (spoiler alert) As we are just getting used to Arran (the leader of the kids we follow throughout the book) he is killed. (end) The remaining characters are terribly flat. There's no character development at all. Things happen, they move around London, they kill grown-ups, and stay the same. Maxie falls for a guy, then another guy. Yawn. It's mind-numbingly dull.
The writing isn't even top-notch. Reading this novel doesn't scare you because of how well the horror is described. Rather, Higson cheaply latches onto "zombies" to attempt to beef up his book. The entire book smells of marketing and cheap ploys to grab fickle readers. This is not a book that will be remembered in ten or even five years.
To top it all off, the novel ends without a resolution: an excuse that many authors these days seem to think will ensure the success of their second novel. Unfortunately, all it does is disillusion readers altogether.
The Enemy is a poor mishmash of cheap tropes, poor writing, bad characters, and an unresolved, if a little bit creative, plot. I finished the book, but if it wasn't free on Barnes and Noble I wouldn't have even picked it up. It's just not worth the time. You're better off getting your action-novel fix in a different place, because The Enemy really falls short.