There are so many challenges in studying Tamil that in a book of this size it is necessary to consider certain trade-offs. The spoken language differs considerably from the written, and it is a very difficult language to pin down to a phonetic system. So what we have here is a book that introduces the alphabet, but has little else to do with writing. Another challenge comes from the speed with which Tamilians usually speak their language; the authors have opted for authenticity, which means the speakers are talking very rapidly indeed, to a point where it is even hard to pick out individual words. One solution might be to buy this book along with Kausalya Hart's 'Tamil for Beginners' (audio should be available from Cal-Berkeley, and the author's recording is at a much easier pace to follow). Also, the Hart book teaches the alphabet completely at the beginning and it is used in all of the subsequent lessons.
Still, 'Colloquial Tamil' is a useful book to have, given the very few resources available for Tamil learners. The dialogues get more interesting as the work moves along, and with a lot of practice listening to the accompanying CDs (sold separately), they will become clearer. A language of instant gratification this is not.