The best thing about this book is that it breaks down the structures and tools found in a screenplay and gives you enough information to feel confident enough to use them yourself.
The examples are clear and all help to illustrate the point.
I found this book extremely useful both for improving an existing screenplay and starting another.
It does not dictate how to write, but teaches you the tricks of the trade so that you can use them in whatever way you wish.
A superb structural reference manual - buy it.