If I had to describe D.J. MacHale's Merchant of Death in one word, that word would be "adequate." It ticks all the checkboxes for YA fantasy: Normal kid discovers he has a greater destiny, alternate worlds alongside ours, helpful but inscrutable mentor who disappears when it's important for the hero to stand on his own, appropriate sidekicks for the hero (one or more of which could end up being romantic interests), super-powered villain who the hero drives away but doesn't defeat (thus setting the stage for sequels). But for all that this book does right, it still felt kind of perfunctory - I felt like I could see the superstructure under the story and see how things were happening because the story structure demanded it, not necessarily because they seemed like they were an integral part of the story itself. In the end, though MacHale held my attention through to the end of the book, he didn't grab enough of my attention for me to want to pick up the sequels and keep reading.
I think the thing that really turned me off this book was the way the background magic/technology just didn't seem rigorous enough. Things seemed to just work with no underlying mechanism in the way that was most convenient for the story. I'm willing to accept any amount of magic or imaginary technology that it takes to make a story work, but I expect it to work within internally consistent rules, not just on authorial whim.