I really liked the fact that the protagonists in this book were around 70 years old. Folks in this age bracket seem to be underrepresented in popular fiction. And I believe King handled their characterizations well. I also liked the surreal feel to the supernatural aspects of this book. Oh, and, I was pleasantly surprised to find that this was a King book with a satisfying ending!
*Note: I'm not sure how enjoyable this book would be for those who have not read the Dark Tower series. It's closely tied to the mythology from those books.