I'm a big fan of this series, and was pleased with the third installment. I was surprised at how quickly things got SUPER Christian - some people say they saw it coming, I did not. In the past two books we've heard plenty about how Elyon created everything and lives in this mystical city, but that felt like it could kind of fit in with any fictionalized mythology / religious system. But book three got super Christian. Fine by me, but if that's not your thing (or if you're a parent and that isn't your thing, and you didn't know when your kid started reading this that was gonna be a HUGE part of it) you might feel a little duped.
And, as a Christian, I think Carman does a good job of weaving Christian elements and values (using those the world deems as weak for an important purpose - Alexa, Yipes, Murphy, Armon, and Odessa are an unlikely bunch).
Craftwise, he could have been more subtle about it. Someone else in their review mentioned things wrapped up too quickly - I didn't feel that way too much. It was a clean wrap-up, though - it felt like the end of the series. The first book felt this way too - that you'd be fine if you stopped reading the series. Whereas the second book left on a major cliffhanger.