I am a committer on the BIRT project, so I am biased. BIRT provides a very well thought out report object model (ROM) and appropriate API for the various design and deploy function that is needed. In addition, BIRT provides the best multi-language support and the ability to separate development from design through the use of CSS.
BIRT can be embedded into your application for no license cost through the REAPI or it can be purchased through a couple of commercial offerings.
