I finished Scott Westerfeld's Leviathan a little while ago, and I haven't returned it to the library, yet.
The reason? I loved the pictures by Keith Thompson.
Yes, this is a fast paced, action filled novel about the heir to the Austro-Hungarian Empire and a young balloonist in the service of England's aeronautics force, set in an alternate Earth at the beginning of World War I. It has cool technology, weird animals, strong characters and an intriguing plot. But more importantly (at least as far as I'm concerned), it has illustrations! Lots of illustrations! Every single chapter has at least one, sometimes more, and they are detailed and exquisite.
You give me enough pretty pictures, and, yes, I'm a very happy person.
On a more objective level, the pictures will make the book more accessible to the younger YA crowd, and very likely reach middle graders who have demonstrated with Harry Potter, their ability to read 440 page books. It's not that the text isn't accessible -- to the contrary. It's a smooth read. Rather, the pictures add an element of fun that kids aren't ashamed of enjoying (and I truly wish adults would get over finding juvenile).
My one complaint is that the book is the beginning of a series, and it doesn't stand alone -- the plot isn't resolved. I resent that in a book. On the other hand, it'll give me an excuse to look at all the new pretty pictures in the next installment. . .