I hate when people compare books to Harry Potter, but this has a similar charme to it and even similar story elements without feeling like a ripoff. Most importantly, it made me feel really similarly.
I love the characters. It's so easy to sympathize with Joel: he's a total nerd and a huge fan of this one thing he will never be able to do: rithmatism. In the beginning, he's at a low, which makes everything to come so much more meaningful. Melody is just awesome: she's annoying to everyone around her but never to the reader. All the minor characters are fantastic, too, and I felt kinda at home in this school.
The writing is so much fun and invoked all of the emotions. I felt sad for characters, laughed out loud, was incredibly proud of them... When I actually talk back at my audio book, you know that I'm invested. :D
Also, the mystery was really good, and while the focus of this book is the personal developement of Joel and Melody and the formation of their friendship (which is precious), Sanderson finds enough time to through in some little twists here and there.
The world building and the magic are awesome, too, and their conveyed in great ways. While before every chapter, the reader is actually taught rithmac rules, the world building needs to be pieced together: how much is this like our world and how did it divert from it? The magic of the book also leads to some genuinely horrifying discriptions that I didn't expect in this book for younger readers.
The ending is really satisfying, but it also makes you crave for a sequel. Will we ever get it? Who knows, but I want it so much.