English Grammar for Students of German has been very helpful to me. I have been studying German for about half a year now, and for the first five months of that I was using Duolingo (if you don't know about Duolingo and you're looking to study German either on your own or otherwise, look it up! It's a great supplement to other German study materials). The problem with Duolingo is that it takes the "natural" approach to learning language, where the grammatical rules are not always explicitly stated or taught. Coming from a computer programming background, and having been raised by an English teacher, the "natural" approach to languages does not cut it for me; I need to know the specifics about languages in order to learn them, which English Grammar for Students of German does.
What English Grammar for Students of German does not do, however, is provide an exhaustive overview of German grammar. That is not exactly a pitfall, though, because a "lighter" introduction to the subject is exactly what I believe most students need, anyway. Besides, the point of this book is to improve student's understanding of German grammar lessons by refreshing the student's English grammar skills, and then contrasting them to German grammar. If you are looking for grammar drills or an exhaustive overview of German grammar, there are several books on Amazon which provide plenty of exercises and more details. You won't find that here, even though I believe this book is a must-have for adult students of German.