Mastering set phrases is definately very essential to achieving fluency in Japanese. The fact that this book focuses on that idea alone makes it worth reading through. But don't expect this book to provide a comprehensive foundation of common set phrases. For one thing, you'd need a much bigger book, and for another, this book focuses almost exclusively on formal, and often stiff Japanese. That isn't to say that the phrases introduced aren't useful, just that they aren't universally so. If you are a diplomat, you'll find the perfect expressions appropriate to making speeches and conducting courtesy calls. If you are an exchange student and use these phrases on your host family and classmates, you'll come off as very polite at best and absurdly stodgy at worst. That said, I do recommend that beginning-intermediate students of the language read this book. The title of the book (for the most part) tells the truth: the phrases introduced are common and therefore it will doyou good to learn them.