Closure Compiler - compiler for JavaScript

What are the benefits of using Closure Compiler? 

Efficiency. The Closure Compiler reduces the size of your JavaScript files and makes them more efficient, helping your application to load faster and reducing your bandwidth needs.
 
Code checking. The Closure Compiler provides warnings for illegal JavaScript and warnings for potentially dangerous operations, helping you to produce JavaScript that is less buggy and and easier to maintain.

How do I start?
http://code.google.com/intl/it/closure/compiler/