It has been an absolute pleasure to read this no-nonsense collection of observations and suggestions.
The author is extremely knowledgeable and articulates his points in a clear, concise and logical presentation, which is a rare characteristic in today's overflowed and low-quality offer of "how-to-become-a-guru" manuals.
The Collections framework is clearly the author's backyard and you will learn about implementation details and rationales that you will not easily find anywhere else.
What I found most useful in the analysis of the various Java aspects was the author's perspective, which is based on the pros and cons of implementation choices, and strongly focused on API construction. Unless you work alone and deliver complete applications, you will define, design and implement an API sooner or later, maybe even without realizing it. With the help from this book you will most certainly design a robust, maintainable and useful API.
I also liked the practical approach that sometimes goes against OO principles: for example, just because Java embraced the OO philosophy does not mean that inheritance is the only way to go. Composition, static factories, singletons, immutable classes and some good old procedural programming are discussed in depth.
Another brilliant characteristic of this guide (and I would like to especially thank the author for this) is that although there are plenty of snippets to illustrate concepts, these are just skeletons, never longer than few lines and therefore they do not force you to waste your time and divert your attention from the core issue by analyzing pages over pages of code when maybe only one line could have served the purpose.
I would say that this book finds its best audience in the experienced developer/architect who does not have a specific Java expertise but is very comfortable with some other programming language. However, everyone can benefit from in-depth explanations of often misunderstood subtleties like the "equals()", "hashCode()", "clone()" or "compareTo()" contracts. Or serialization: are you sure you master it?
You don't need a profound Java working experience to immediately grasp some concepts; I found that this guide was an excellent companion in my learning of Java, you can start reading it from day 0, and you will get back to it every time you want to know more about a new concept.
The best praise I can say about "Effective Java" is that in my opinion only a handful of experts in the whole Java community could rightfully say "This book shall not teach me no thing", and then I would like to work with these people.