To premise my review, I am a person who is living and working in Korea and studies Korean by myself in my free time. I don't take after work classes since I haven't found a school near me that teaches Korean in the communicative method.
That being said, the reason that I am giving this book 5 stars is the explanation of the different grammar points are extremely clear. I have found that most text books (even those from the major universities) are extremely lacking when it comes to explanations of grammatical points. This book, however, gives clear explanations in natural English (no poorly translated Konglish phrases). The people who wrote these explanations seem to have English speakers in mind. In addition to this, there are quite a few sections within the book that gives wonderful comparisons about different grammatical patterns that are very similar (such as 이/가 vs. 은/는 or 아/어서 vs. (으)니까). Even though I have studied Korean for quite a long time, there were a lot of 'Ah ha!' moments for me in this book.
I wouldn't use this book solely to learn Korean. Once you get a strong enough base in the language, I would keep this book around to supplement what you are learning in other textbooks. Any time you get to a new grammar point and the book or your teacher does a poor job explaining, this is the book for you.
The weakest aspect of this book is that the practice is mostly fill in the blank exercises. There aren't any activities to help you practice the language in real world situations. Despite this shortcoming, the clear explanations of Korean grammar is what really makes me love this book.