I see this book first and foremost as a coming-of-age and love story, and then a science fiction. The science fiction aspect takes a backseat to the romantic plot and that's perfectly fine, since it really is the least interesting out of everything in the book.
David is a great protagonist, very well characterized and likeable, and you can really see how he grows and matures as the story goes on. Callan pales in comparison, and I felt his character was never as developed as David's was. I liked the way their relationship developed, it was very sweet and believable.
The only disappointing thing is that the questions revolving the dragons and basically all the science fiction elements were never answered despite that they seemed like such an important plot point throughout the novel. I understand there's supposed to be a sequel so maybe these things will get resolved then. The romantic relationship, however, does get fully resolved by the end and it was such an engrossing and excellent read that I'm very satisfied overall.