First off, I want to say that I would not recommend this book for casual students, or people who want to study a bit of Thai for a holiday. This book is people who are serious about learning the Thai language correctly. (For those going on holiday, I also strongly recommend the phrase book from Lonely Planet.)
Second off, I want to say a bit about a few 1 star ratings I see. One complains that the book does not come with the tapes or a CD. Another complains that the tapes don't come with a book. Well, yes, this is true. If you want both, you buy both. What in the world does this have to do with the quality of either, and how does it justify a one star rating? It may be a legitimate complaint that buying both is too expensive (I would disagree), but these kinds of complaints are like saying that DirectTV satellite TV is awful because it does not come with a TV to watch it on.
In my opinion as a fluent Thai speaker, who has looked at many systems to learn the language, this one is the best. Here is why:
1) It has you learning to read and write right from the beginning, and if you're serious about learning Thai I cannot emphasize strongly enough the importance of this. It will aid your pronunciation and understanding to the language immensely, and in the grand scheme of learning the language, learning the writing system is actually a small part.
2) While you're getting up to speed on the writing system, it's transliteration system is there to help you, and is the best in existence. It has an extended roman alphabet to accurately and consistently represent the Thai alphabet. You need to take a bit of time to learn the system, but the payoff is you will be leaps and bounds ahead of students of other systems as far as pronunciaton goes.
3) It presents the major grammatical elements of Thai in a well pased, logical way.
4) The vocabulary building is also very good, introducing common stuctural words quickly. Vocabulary is introduced in a context so many realated words can be learned at once.
I recommend this series to anyone who has an interest in learning Thai.