This book helped me (and continues to help me) clear up a lot of confusion I had with certain aspects of grammar throughout my entire autodidactic German-learning process. Its textbook style format is best suited for those who learn from having things explained logically and analytically, as opposed to those who learn from observation or drilling methods. I don't recommend it for beginners as their first German language book, rather as a reference to be used as you progress through the intermediate levels. It is indeed useful to beginners as a general reference, though delving deeply into the lessons would be a headache for most who haven't progressed past beginner levels. The book is very concise and gives lengthy explanations and examples, so you're sure to understand the information given as long as you pace yourself and are a studious type of person. It even includes the most recent updates (and those known to come) to the language as of the published date (1996). Having not yet gone through the Fourth Edition I'm unable to review it, though I believe it to be equal to the Third as far as the learning format, and presumably the most recent updates to German as of 2011 have been included as well.
Of course no one book nor one learning method should be used in the learning process, but as far as having a concise manual in which to refer to while you learn as a beginner or progress through intermediate levels, I think this book (or better even the updated Fourth Edition from 2011) is ideal. Advisably I don't recommended trying to learn any language quickly, and most books claiming to help you do so should be avoided, from my experience most especially with German as a native English-speaker.