If you don't have a friend who can speak Cantonese, then don't buy this book - but do buy the Book + Audio Cassette version instead.
Used on its own, i.e. without the cassette, it provides a good lesson structure which you can follow with the help of a Cantonese speaking friend. I think that knowing a native speaker is pretty much a requirement to learn such a precise language anyway, as a small change in pronounciation makes a whole load of difference to the meaning.
The book uses funny dialogs and cartoons to help you learn, and also has exercises in each chapter to make certain you have understood everything that was taught. For example, making a meaningful sentance out of a 'jubbled up' one. To further check your progress there are revision chapters throughout the book, which are there to consolidate all of what you have been taught so far.
In summary, this book will not do a brilliant job of teaching you to speak Cantonese IF you intend to use it on its own. BUT like I've said, if you have a Cantonese speaking friend to teach you pronounciation then it provides a very good lesson plan for you to work from.