I am doing some work where we decided to use a rules engine. We are on a java/hibernate/struts stack. Are leaning towards Drools as it has some integration points with AppFuse that we are using. Plus FICO Blaze is way more expensive
Drools/JBossRules is actually pretty sweet (disclaimer, I worked on it once upon a time and JBoss gave me some kick ass headphones when they bought it from Bob and Mark). They have been taking it more in the direction of a traditional business rules management mabob, and I don't love their little language personally, but it is WAY nicer for programmers than Blaze, plus the devs are always in IRC and helpful, cannot beat that for support.