I finally finished this tome of grammatical knowledge after almost two months of reading 10 pages a day.
In no way does reading this book cover to cover make me an expert on grammar, but I did learn a lot of little tidbits of information to help explain better certain errors that I've corrected my students on.
This book was a good overview, but didn't go into too much detail. There were some points that I was able to understand quite easily after reading the example sentences that I feel a non-native speaker would have quite a bit of trouble understand without a more thorough explanation.
During my CELTA course, when I was first using this book, I found this book was good to use first when looking up certain grammar points. This book gave a nice overview and basic understand of the point being made. Afterwards, I would consult other grammar books for more in depth details and be able to understand it better.
As an ESL teacher, I can surely say this is the best grammar book I have ever used. I got familiar with it when I was still learning English. The way grammar lessons are taught are super easy for learners, using various illustrations to make things more understandable for learners.