In typical (and awesome) Sanderson style, the creation of the alternate world for this book is deep, layered, fascinating, original and full of surprises. It's also written as though you already know about all of it, rather than building everything in the first few chapters. This is what he did with "The Way of Kings", too, and I felt the same slow draw to get into the story with that book. Once I did get into the story, however, I was hooked. Particularly at the end. (In both cases!)
Fortunately, Sanderson's worlds are excellent, and his characters feel very real. Joel feels familiar, like I've talked to or known him before. His friend, Melody, is unique, but not in a crazy, inauthentic way. I appreciated that the bad guy wasn't who I suspected, and I liked the ending, which left many questions unanswered and set up the next book nicely.
The steampunk, gear and gaslight world is kind of fun, and allows for the chalk-based mysterious magic to work well. Clockwork crabs cut the grass, and lanterns have to be wound up to work, and people wear cloaks and bowler hats.
The drawings in the book are lovely-- very well done, though I couldn't begin to tell you which rithmatic defenses are which (Sanderson loves a complex magic system!) even though they are well described and diagrammed. My brain can't hold on to that kind of information for very long.
Overall, a good Youth Fantasy Fiction book. I took a lot longer to get into it than I expected to, but once there, it was quite a fun read.