Kinda torn on how to review this... I think it's a great read if you go into it with the right expectations. On the one hand, you can't expect a thrilling plot or complex character development or impressive prose. The world and characters are all super flat. But I don't think the author was too concerned with that when writing the book. In some ways, it even helps to emphasize his point of God's sovereignty.
On the other hand, the book made for a really fun, simple read (Name a more classic archetype than a boy going on a quest to slay a dragon and rescue a girl. It's what Christ does in the Bible, after all.), with all sorts of deeper meaning sprinkled throughout. As other reviews have said, it's a hearty mix of Tolkien, Lewis, Bunyan, and Chesterton. It's the kind of book I'd like to have read as a kid — the kind that tells children that dragons can be killed (see the famous G.K. Chesterton quote about fairy tales). The author is also a bit of a wordsmith, which made for some fun one-liners.
If the number of quotes I highlighted is any indication, this is a solid book. Here's a sampling:
"Finally, the dragon spoke, and his voice was a mountain brook clattering down over a steep jumble of rocks."
"His white hide shimmered in the morning light, as though his color was the sum of all the colors, and not the absence of them."
"The dragon is evil, but not an outlaw. He follows the deepness of things and loves to do what is expected of him."
"So you may not think you have the strength, or power, or wisdom to attack Fafnir. And this is all true. But you do have the authority."
"The creature seemed rebellious and evil without having rebelled against its own nature."
"He said that it was like breathing glory, and that it was too thick for him to breathe in."
"The dragon obeys all the rules of the story. He hates them, but he obeys them. If you ever meet a dragon in a story that does not obey the rules, then that story is even more wicked than the dragon is." (even the White Witch in Narnia has to abide by the deep magic, even if she doesn't understand it)
"'Who brought you to this place? The one who writes all stories,' Kyru said. 'We don’t know how, or fully why, but we can always know who.'"
"But when the dragon is speaking, it will seem to you then that this bread was made from sawdust. Our words to you will seem full of malice. The water in this pitcher will be remembered as foul. And when you enter that great darkness, do not doubt in the dark what you knew in the light."