Deep in a thread about using gcj as a portable way to write programs on different architectures someone mentioned PyLucene, a python integration of Lucene (a full-featured text search engine library written in java), using gcj and SWIG. Which is a really nice showcase for what is possible with GNU gcj. It is used and developed by the OSAF for Chandler which is mostly written in Python.
When submitting the integration to the Lucene project Andi Vajda said:
Yes, I took a look at Lupy [python port of Lucene]. It claims to be 10 times slower than the java version. There is also a C++ port of Lucene, which would have been simpler to integrate, but it comes with its own set of bugs. And of course, each port is behind.
Which is why the fact that GNU gcj could be used to combine java source with Python (through Swig) is so nice. It (partly) solves having to rewrite something in another programming language just to integrate it into a larger project that has a different language preference.