I was a bit nervous when I picked this up, as it is quite thick, but it is actually a very quick read. The story flows nicely, and part of the bulk is due to think paper and gorgeous illustrations. Westerfeld commissioned the artwork himself which gave him much more control than authors usually have with this sort of thing. The illustrations really add to the story, it is an alternate history where you have 'Clankers' (who make really big, clanky, steampunk machines) and 'Darwinists' (who have taken genetic engineering in some really crazy directions) and both are far more understandable with the illustrations than they would be otherwise.
The other thing I loved about this was the two main characters. They are both children who are forced into adult situations. This is done very believably, and I think most kids would be able to empathize with them. The setting is quite interesting on its own, but the book is action-packed and easy to read (which is possibly why I like YA so much).
I've enjoyed all the books by Scott Westerfeld that I've read, but I think this is my favorite so far. I'm definitely looking forward to the sequel!