Many of this book's translation exercises were very helpful to me-- they would often draw on material a couple of chapters old to review, forcing me to re-check my understanding of concepts I hadn't even realize I hadn't absorbed. The single-sentence translations and short writing assignments were a particularly helpful way to prepare for tests if I hadn't already completed them.
However, the book also contained exercises that really only tested knowledge of the cute little storyline being developed in the book's dialogs. While these tested reading comprehension sometimes, other exercises require one to actually go back and re-read the dialog to find the exact word and conjugation used, rather than to prove an understanding of the text. Other exercises waste space with unnecessary illustrations, or are poorly formatted-- for example, they may depend upon an illustration on the opposite side of the page.
These, of course, are minor problems present in almost any foreign language workbook-- the real problem that makes this book a poor buy is that the answers are not provided in the back. I might be able to use this book to practice a concept I didn't understand, but without a fluent speaker to correct it for me, the exercise would have been useless-- someone attempting to self-teach could easily make the same mistakes over and over throughout the entire workbook, wasting their time and reinforcing their learning of an error. If this book is required by a class, of course, you have no choice, and professors will probably work around this problem by correcting work themselves or together in class. However, if you're attempting to teach yourself, the lack of answers makes this book practically worthless.