The caveat is that sometimes the information recorded in the metadata is missing or incorrect.
Use the information on our Components page to determine which jar files to include in your classpath. So if you're motivated, ready, and have the time: Download the source from the Subversion Repository, build the code, join the mailing lists, and we'll be happy to help you get started on the project!
As a general policy we collaborate as much as possible with other projects to provide this functionality.
Examples include: Cocoon for which there are serializers for HSSF; Open with whom we collaborate in documenting the XLS format; and Tika / Lucene, for which we provide format interpretors.
When practical, we donate components directly to those projects for POI-enabling them.
A major use of the Apache POI api is for Text Extraction applications such as web spiders, index builders, and content management systems. You'd use POIFS if you had a document written in OLE 2 Compound Document Format, probably written using MFC, that you needed to read in Java.