Geoffrey Wiseman at InfoQ - Real-World Rule Engines:

"For many developers, rule engines are buzzwords, or black boxes on an architectural diagram: something to be feared or admired from afar, but not understood. [...] Over the course of this article, I'll be sharing my practical experience with rule engines and with Drools in particular to support in-market solutions for financial services, in order to help you understand where rule engines are useful and how to apply them best to the problems you face.

[...] Although rule engines can be applied to just about any problem if you try hard enough, they are simply another tool in the toolbox. They don't replace constraint programming and solvers, artificial intelligence, workflow engines, decision tables or general-purpose programming languages; they simply supplement that set of tools with another."