BIRT is part of Eclipse. It has a relaxed license in the form of the Eclipse Public License. As it is part of Eclipse, it has the ability to work with other Eclipse tools inside of a project. BIRT reports are simple, structured, human readable XML documents. When deploying BIRT, there are several options due to BIRT's flexible structure. It can be deployed in a standalone Java application, inside a J2EE application, and inside an OSGi/RCP application framework. It comes with a good set of tools to get you started such as a graphical report editor and an easily deployed report viewer for J2EE applications. BIRT Reports also do not require compiling. They are run and rendered from the XML report design file. The BIRT framework is also extensible. There are built-in extension points that allow application developers to add in new features such as new data sources, new rendered report output types, and new aggregations and report component types. And yes, BIRT is backed by some big players such as IBM and Actuate.
However, the thing that keeps me involved with BIRT is not the technical side. The thing that keeps me involved with BIRT is the community. There has been a very strong community built up around BIRT, centered around the BIRT Exchange and the BIRT news groups. The people who manage and maintain these communities work hard to provide the BIRT users with good examples, answering questions, and promoting the use of BIRT in their products. Users are always helping each other out and I have seen this community grow significantly over the past several years.