It's a fast-paced easy book to read and can easily be consumed in one sitting. There's nothing too gory or too racy on the page so it is appropriate for readers of a wide age group. However, it won't appeal to everyone because it pretty much is alternate worlds hard core quantum physics sci-fi stuff and not your typical Y.A. story. It's very well written and it's easy to sympathize with Callum's plight. He finds himself the terror of his local high school and a sports star when in his real life he's something of a nerd. While the story is generally fast-paced, I found the balance of the plot a little "off" in the sense that the story takes a good long while to get going, focusing most of its approximately 300 pages on the differences between the worlds, and only gets to the meat of the problem (i.e. how Callum comes to be where he is) at the very end of the story. So the end was a little unsatisfying for me in terms of balance, although in terms of tying up loose ends, Blackwell does an excellent job.
While the story has a male protagonist and is kinda "boy focused" in terms of fights, sports etc, I'm pretty sure it would appeal to female readers too. Hey I'm a girl and I liked it. So if you're looking for something a little different in the Y.A. sci-fi realm, this isn't a bad option for a change from all those vampires, werewolves and angels ...