For fear of ruining the story for anyone all I will say is that:
1) Stephen King does a great job of bringing the past to life
2) He manages to give the Oswalds humanity
3) He makes a lot of plugs for the value of libraries. They were integral to research in the past, offer aid to millions of people now, and will be extremely important in the future as well.
Really liked it. I hope Grossman is planning to make this into a series similar to the Narnia books. If you're going to be the C.S. Lewis for adults it would be fitting to make the series last around seven books. Of course, the books could end with this one and it would be fine, but I would love it for Ember or some other demigod to call on Quentin again. Or it could take place on Earth. I don't even care where the books are set as long as they keep coming.
This is also a good storytime book, particularly in the winter.
This is a good choice for reading at a storytime. There isn't a lot of dialogue or explanation so you can ask the kids what they think is going on, do funny voices for the animals, etc.
It was good, but I can't forgive it for depicting a Joker origin story (even if it was a hallucinated past). The Joker is best left unexplained.