After the past few middle school targeted books I've read, I was worried that Max and the Gatekeeper would be the same recycled plot and characters. It was a nice surprise to find, instead, a really well-done book. I mean, sure, the plot is nothing original. But there was enough care put into building the worlds and the characters in this story that I can overlook the plot.
The creature design was what impressed me the most. This book didn't go for the easy creatures (although one of the character is, technically, an elf) - time was actually spent making unique animals and aliens, and building their worlds. It made me think a lot of the Pendragon series, if Pendragon was written for a younger audience and had less humanoids occupying the various worlds.
The antagonists were pretty stereotypical. Their end goal is domination of all the worlds. However, the villains get credit for supplying an explanation for the bullies that show up early on in the book. Thank you for that. Bullies are fine in a story, so long as they have actual motivation behind their actions. And I'm pretty sure "our parents work for a demon master, and we've got to kill your family to get what we want" is a good excuse for a bit of name-calling and fisticuffs.
Max was very well developed over the course of the book, and was one of the more realistic middle school protagonists I've seen in a while. He also didn't overreact to situations, which a lot of these stories tend to do. He starts off unsure of himself, and scared of having to eventually save the world. He hates that this fate is basically being pushed on him. It's a pretty normal reaction for a kid to have under those circumstances, and he actually handled it very well, considering. As he gets trained, and as he sees what's at stake, he finds out that he's stronger than he gives himself credit for, and he's willing to lay down his life to save his friends and their worlds. That, or the villains are all wusses, which is entirely possible as their plans all get ruined by two twelve-year-old children.
Cindy was a nice sidekick. She wasn't a damsel in distress, but she also wasn't a complete tomboy, to the point where you wonder why she wasn't just written as a male character.