I've studied German, Spanish and French to varying levels. I have a collection of 20 or so language books, and I can honestly say that this is the best.
The reason is simple - whenever you learn a language you end up speaking in an unnatural, stitled kind of way. That's certainly how I was taught at school, and that's how most language books do it too. I remember the first time I was in Germany, someone would ask me if I liked a meal, I'd say something like "I am of the opinion that meal was delicious". Who talks like that? But that was the only way I knew how to talk, because that was how I'd learned.
I returned from 6 months in South America speaking Spanish wanting to take my Spanish to the next step - to get a snappy kind of colloquial speaking that normally only comes with practice. (just to echo what's been said before, this book is for intermediates, not beginners, though they might find it fun too).
This book will teach you how to avoid common mistakes, snappy phrases, comebacks, and most importantly of all HOW WORDS ARE ACTUALLY USED. If you look up a word in a dictionary, even a very big dictionary, the context they give you seems to be guaranteed to be wrong in some way. Here, the context will be explained to you in plain English.
I found the book helped me to think in Spanish better, because you need phrases like "errrr", and a few choice insults probably when you're thinking in English. I have read this book cover to cover about ten times. I even bought another copy myself, just to give to a friend because I think everyone should have this book.