How many people have actually implemented Rules Engines? I first ran into Expert Learning Systems in the 90's when working in the Mechanical Computer Aided Engineering (MCAE) space. They were being used to electronically document engineers/designers knowledge for the development of mechanical components. The idea at the time was to use this knowledge to optimize part development and reduce lead time. The infrastructure behind this was a Rules Engine.
I don't know the answer on the current adoption of Rules Engines but for the right problem, it appears that it provides a compelling solution. I am currently investigating this sector to help identify if the technology is pluggable in a SOA and what type of business problems are best served. It appears that logic that is dynamic is the best candidate. The ability to update an external configuration without having to go through a typical release cycle if the logic was embedded in a bean is definitely attractive. The languages are pretty different between vendors, some using Natural Language approaches while others use a proprietary scripting language. The only constant is support for the JSR-094 specification.
Most of the vendors generate a java implementation or even web services so inclusion into a Web Services infrastructure is pretty straight forward.
So far I have looked at Jess and OpenRules and found the open source community to provide a very good starting point.