For example, say I use Struts in most of the projects I'm working on. Instead of defining a CLASSPATH, Source Path and Javadoc Path for Struts in each project I set up, I simply define it as a library. I can then choose to include the library as part of my project.

I keep a copy of struts.jar in a common directory, and have each project's build script take a copy of it when building.

(The only problem I've found with this is if you're dealing with multiple versions of the same library. You must make sure that each build script copies the correct version of the library it needs.)

On the other hand, it keeps the number of JAR files way down, since you don't need a separate source JAR for every project.

I know some developers who never use the Library feature. IDEA makes it so easy to set up libaries as part of the CLASSPATH that they don't bother.