It's nice to read a book that is original, and Scott Westerfeld has done it again. I would even go so far as to say that this series looks to be even more promising than his Uglies series.
I will admit, I at first had my reservations about Deryn's storyline, as the 'girl dresses as boy to get into army' thing has been done so many times before, but I soon warmed to it. What redeemed this story, in my eyes, is that she is not a typical female character: she doesn't whine about the men being unfair to her (or treating her the same as everyone else); and she doesn't go all angst-y about whether or not to tell everybody (this only pops up a couple of times, and not for long).
In the end, I got so into the novel that I was practically screaming at it when Deryn decided to capture Alek, and so I believe that a book so able to draw me in well deserves its five stars.