It works for me for generating PDF for quite some time. I get NPE when reloading a FO source in the AWT appilcation, but this maz have other reasons, I didn't try to track it down.
As long as I can remember I got NPE when clicking the [Reload] button in AWT while running FOP (w fop-0.20.5 & fop-0.20.4). I just took it as "fact" that this was something we're not supposed to do.
Web Maestro Clay