Very useful. The words defined in this book do come up at the TOEFL exam. I entirely read it an hour before the exam and I was glad I did because it helped me for a couple of questions on the reading part of the test.
This book provides a nice structure to learn vocabulary; it groups the words making it easier to memorize them as well as apply them (fluency). I strongly reccomend this book as a basic jumping off point to students and teachers.