I am a professional English teacher, and have used this book with two of my students who are preparing for the TOEFL. The level of the book is kind of upper-intermediate, and the 400 words are quite useful, and your students will encounter many of them again later when they take the real TOEFL. The headwords are explained well and become even more understandable after reading the example sentences. Also, different parts of speech are given.
Sometimes it seems to me that some of the example sentences which are used to illustrate the words' use require either having lived in the US for some time ("As usual, their holidays were ruined when their in-laws' views on politics collided with their own."- In my experience not many foreign students know about the US tradition of arguing about politics during Thanksgiving and Christmas), or heavy use of a dictionary ("Before injecting a painkiller, the dentist rubbed CLOVES on a woman's GUMS to NUMB them."- I don't think things should be dumbed down, but these kinds of sentences limit the usefulness of the book for self-study), or simply a teacher to stand there and explain to you the context behind such sentences. Another nice thing is that the units are short- in self study, you could go through a unit in 35 minutes and learn 10 new words with good example sentences.
To sum up, it is a good introduction to the kind of words you will see on the TOEFL if your level is not quite enough to independently sit down and start with practice tests, but is no substitute for the practice tests themselves. If your level is high enough, you may want to just take the bull by the horns.
I hate to close with a complaint, but the cover of the book proclaims that there is a complete review of roots, prefixes, suffixes and more, and there is no such thing in this book. For that reason I subtract one star, and give this book a three.