Not perfect, but - especially the second half of the book - exciting and thrilling.
The characters, especially Alek, could have used some more dimensions. I liked Deryn (although more in the first half of the book - the way she overdid the whole pretending-to-be-a-boy thing started to get a bit wearing later on, although I understood the need), but Alek was just such a cliché, cardboard character with zero personality for the first half of the book.
Fortunately that got a bit better, and the exciting plot made up for the lack of characterisation.
I can't say I'm thrilled about the "subtle" hints pointing at what seems to be an eventual inevitable Deryn/Alek pairing, but I'll withhold judgement on that until I've read the next books.