So far, I've been happy with it. I just started using the book, and it is helping me reinforce some previous practice of the alphabet by introducing vocab a little at a time. In the first topic, there is a mistake that stood out to me a lot they had the name "Helen" not as '', but with ' in place of the first '. The flashcards at the back at the book (albeit nice to help with the learning process) have this same mistake, but the Hangul for "Helen" and "Mary" or on each other's card (Helen is still wrong on it). The card with "Minjun" in Hangul also has "Mimsu" on the front. Other than that, I haven't noticed any other mistakes. So far, I've found the amount of exercises, vocab, and language information given to be efficient for learning it all; not too much and too little.
I'll try to update if I notice any more mistakes.