I really enjoyed the book, helped me to understand the decisions made when the Dart language was created. The examples are compact enough to quickly understand, and it doesn't have too much of a marketing message some books on new programming languages have. I began coding Dart once I started reading this book, mainly small things for hobby projects. Now using Dart for a larger project for the first time. The book gave me a strong foundation to be able to understand the Dart as a language in a much better way. Some of the newer Dart developments in 2016 like "strong mode" and AOT compilation naturally could not be covered - since those features were not available at the time of the writing of the book.
Unlike what the cover text says, this book is neither very rigorous, nor readable. In the end, you get the idea, but I wouldn't say it does a good job explaining all the choices the authors of the language made (except for the recurring "it was too hard, so we hacked together something that appears to work but really doesn't"). Overall, it feels much more like a collection of blog posts, rather than an explanation of a programming language.
And as for Dart, the language, this book brought me from an initial excitement to disappointment without having to spend too much time with it.