I liked this book. Coupled with Syd Field's Screenwriter's Workbook, I managed to write a first draft of a screenplay. I've never been able to complete a play or screenplay before reading these books! This book gives you the background of screenplays and writing, plus his theory of what makes a good Hollywood screenplay. The workbook gives you a step by step process of writing one.
One drawback is that this book was written in the 80's. Sometimes it sounds so dated. The other drawback is it only explains one type of screenplay, the standard Hollywood 3-act narrative.
Overall, this book was a great help in writing a readable well structured screenplay.