I love this book! I have this in hardback, and am currently reading it for the second time (and I've pre-ordered the paperback of this and the sequel, Steelstriker).
Marie Lu has a way of creating characters who've been through heartbreaking trauma who rise above their circumstances to become heroic.
While this is not part of the first impression... the end! I finished the book at three in the morning, and it left me speechless.
I haven't met a Marie Lu book I haven't liked. Get this one, and try her other books too!