Personally, I can't think of a single flaw in this book. Having taken roughly four years to create, you can really see the care Dillon poured into this book.
Dillon's watercolours are amazing - loose enough to create a flow, detailed enough to make you wonder how he whipped all of these out, and a delicate palette to create a softness while reflecting the mood of respective scenes. Even the digitally inked panels, while flat and bold, have a freshness and his strokes bring them to life. I was left staring at each panel, and sometimes found myself lingering on the same panel for ten minutes.
Having lived in London my whole life, I know quite well most of the settings Dillon chooses. He creates characters that are all fully developed and recognizable, and most importantly - believable. That's one of the best things about this book, I felt like I could go out and possibly bump into Nao on the street.
Dillon chooses to weave two storylines that reflect eachother in some way, and differentiates with clear art styles - the main, of course, being about Nao. While I haven't personally shared Nao's experiences, when you get down to the basics of emotion we can all relate in some way, and I quickly found myself attached and sympathizing with her.
Overall, I'd say this is a subtle book in many ways, and the art alone is reason enough to buy it. Luckily for us, Dillon's as skilled a writer as he is an artist and you'll really want to take this journey with Nao.