The official answer comes from the copy on the back of the book:
For new users, this book is a logically organized and clearly expressed introduction to a big subject. For veterans, it is also an invaluable guide to the expert techniques they need to know to draw a lot more power out of this incredible tool. Your get a broad overview and a deep understanding of the features in IDEA.
Let me also suppliment with my personal opinion, and detail two groups I feel could glean major benefit from the book.
One: developers learning IntelliJ IDEA for the first time. For the developer who's hesitant to play with a new IDE, the book gives a comprehensive overview of all the major features that most developers find useful on a day-to-day basis. They can walk through the sample exercise - a simple currency converter application - and get a holistic understanding of project scructure and management, building, running, debugging, unit testing, J2EE integration, web app front-end development, version control integration, etc. If they're not familiar with an IDE yet, it's an introduction; if they are familiar with NetBeans or Eclipse, the book illustrates some solid features for comparison.
Two: senior developers who need a reference into specific features. For the developer who's comfortable working in IDEA already, we've covered in each chapter a breadth of features that you might not have run into organically. I know Duane and I used the editor for the better part of two years, and were constantly asking each other, "What was that? What did you just do there?" as we saw new menu options or key combinations that we had never personally used before. As a final example... if you've done a lot of J2EE work, but never used IDEA's GUI builder, the book would be a very handy reference for ramping up on it quickly.
Cheers,
Stephen