Most of the concepts in the book are well explained, though I did find myself needing to re-read certain sections twice to completely understand the topic. This is in part because the book is very short, coming in at 153 pages (including the index). This is a double edged sword in that it allows you to take in a lot of material in a short amount of time, but sometimes you’re left wishing that a little more time was spent on a certain topic.
Overall I really liked this book. It gives good coverage to some important topics regarding objects and functions, and gives a good explanation of prototypal inheritance. A lot of language quarks are also pointed out and explained. For example, one thing I didn’t realize was that the arguments array that is implicitly passed to functions isn’t a real array. It’s just an object with a length property. The appendix sections covering the “bad parts” and “awful parts” were also pretty interesting, though I disagree with the author that ++ and — are bad.
If you’re interested, the book’s author gave a one hour talk on this book that touches on a lot of its ideas. You can watch it here (actual talk starts at 2 minutes 15 seconds):