I've been using docx4j, specifically the pptx4j part, for a few days now, and it has been pretty impressive so far.
I wish I could remember the exact problem, but something was omitted from the 2.6 release, and was quickly patched. (I can go back and try to reproduce the situation, if that is important). PowerPoint manipulation was not working for me (or anyone?) before the patch, but the patch fixed the problem. That's good... ...