This gets 5 stars, because it has 100 math formulas in the back. Most people will not have to memorize all 100 formulas, because many are already memorized in students' minds!
The rest of the book was decent, and I think it gives some good tips for the student planning on taking the ACT.

Well, I was expecting something that would let me gauge the difficulty of the ACTs and the practice tests in this book, didn't really cut it.

First of all the English and Science problems were way too easy in this book. It gave me a false sense of security as far as timing was concerned. Actually that was my main problem with this book, so it would be best try to finish these tests with at least 5 minutes left.

Will be used by my 11th grader at school, was a mandatory
purchase to be used during class.

This is an excellent guide to the ACT. It provides you all the information you need to succeed. It gives you strategies that take advantage of the test structure. It also shows you how to best manage your time. The book gives you enough material in the Math glossary and 100 Key Math Concepts to quickly teach yourself the math you need so you will test well.

The book provides two full-length ACT practice exams. Additionally, it gives you free access to many more online tests at Kaplan's website. Anyone who has the discipline to read and study a good part of this book (including the math sections) and do at least one of the book's practice exams is bound to do a lot better on the ACT then if they had not prepared for this exam. Overall, I think the ACT is less studied and coached than the SAT. This represents an opportunity for the ACT test takers who do get prepared by buying this book.