This book very much lives up to its promise as a 'colloquial' course. Like with many languages, there are considerable differences between formal, literary Persian and the everyday spoken idiom. Unlike most courses, which tend to focus on written standards however, this course is strongly focused on the spoken form which has both advantages and disadvantages depending on what your interest in Persian is. If you are primarily interested in reading the works of Rumi, Hafez or Omar Khayyam in the original, this course is not for you and you should give it a wide berth as you will probably find it bewildering and unhelpful. I would recommend instead Narguess Farzad's 'Teach Yourself Modern Persian', and/or the more scholarly Wheeler M. Thackston's 'introduction To Modern Persian' (both of which also have brief, slightly perfunctory guides to spoken Persian included if you are curious).
If your main interest however, is conversing with Iranians, visiting Iran for whatever reason or trying to understand the dialogue of Abbas Kiarostami's films, then you cannot do better than this course. The essential difference between spoken and literary Persian revolves mainly around pronunciation (which can be strikingly different at times) and grammar (as might be expected, the spoken grammar is simpler than the literary standard). Just as an example of the variation involved, a sentence in literary Persian such as Aan mard daaneshju-ye tahrani ast va be Aamrikaa mikhaahad beravad (That man is a Tehrani student and he wants to go to America) would be Un mard duneshju-ye taruni e o Aamrikaa mikhaad bere. Quite different, even at first glance.
In general, this is an excellent course in the spoken language, unfortunately it can waver, at times inconsistently, between literary and colloquial forms- a problem that also occurs in the Persian script. As I understand it, most writing is done in literary rather than colloquial Persian so a more consistent policy towards how to present spelling and written grammar in Persian script would have been helpful. I would recommend this course but I would also suggest that serious students invest in 'Teach Yourself Modern Persian' and/or proffessor Thackston's book as supplementary texts and to help better navigate the subtleties of this rich and beautiful language's finer points of usage.