I have attended a very nice session at TCJUG back in November 2008( I know I am very late in reporting that).
The speaker DJ Des Jardins made a very strong case for JRules. While there are many open source rule engines available in the market like Drools, JRules stands apart from all those because of its capabilities. If you see the presentation available at the site you can judge by yourself that JRules is simply superior compared to other rule engines when it comes to customization, options and flexibility. Check that out.
However, the slides are not very elaborate. To appreciate the full capabilities of JRules you may want to try it once.