Before going into much detail, the most important thing to know, in a review of a children's book, is has the child asked to have this read again? Indeed, our little boy has, more than once; which may be all you need to know.
Other than that, the story, though in a well-worn theme, is a thoughtful take on the idea of the imaginative potential of simple objects. This isn't to say that the author lectures the reader on 'the-importance-of-imagination', rather that he reflects back to children, in a way they'd relate to, something of their own nature.
Hegley is a poet and this comes through in his prose, there is a rhythm to the words that I recognized from seeing him perform. It is somewhat different to the usual children's book style, though it does have a lovely lyrical quality.
The illustration may divide people due to its deliberately naive style, it's a style that I appreciate requires a great deal of practise (it's not actually that easy), though it's not one that I enjoy. That said my boy (two and a half) did enjoy it, he talked animatedly about certain pictures and showed a great deal of interest in it.
Despite my own prejudices about the illustration (I must be getting old, it'll be oil paintings of Lancaster bombers for me next), I and my boy both enjoyed reading this together.