Disclaimer 1: the rating is for the book, not the technology
Disclaimer 2: this is an "old-fashioned" learn-the-programming-language kind of book, close to previous books by Eckel (e.g. Thinking in Java) - in my case it's a pro, but I'm an old-timer
My context: this was my first experience with Kotlin. I reached out for this book because I frequently need to do something on JVM - with Java interoperability, but I don't want to use Java itself (because of my individual preferences). I used to solve that problem with Scala (which I've initially fallen in love with), but that language has its issues which unfortunately will probably never be solved. Kotlin is gaining popularity, it's very readable, it was designed by engineers not scientists - initially, I thought it's good only for mobile development, but it seems it's a very solid general-purpose language.
I like AK's simplicity and clarity. Each chapter represents a construct - a new building block. Each chapter assumes you're familiar with the content of the previous ones (in general). They are short, but meaty, no diagrams, but really good, focused code samples. All the most important aspects of the language (AFAIK) have been covered. So is there anything I didn't like?
1. there's not much about the Kotlin ecosystem there - Kotlin-specific libraries, frameworks, tools, IDE support, add-ons, etc.
2. there's not much about Kotlin roadmap and language development - where are the things heading, what to expect, etc.
3. I've expected even more detail on Java interop - yes, there are some meaty details (e.g. on generic implementation and how it corresponds to Java's one) and the basic integration scenarios are covered, but I expected more "real-life" advice
4. If you're a fan of "modern" programming language learning books (no theory, learning by doing), feel free to subtract one start from the rating.
That all makes the book very, very good. 4.5 stars good, rounded up to 5.