We've used an Java-embeddable Prolog engine called tuProlog to provide the reasoning capability in several applications.
It's a very nice little language for rule processing, and it has served our needs well. It is unlike most other programming languages in that it's declarative as opposed to procedural, and that can take some getting used to. Our primary motivations for choosing it at the time were that (1) it was a "rules" language with which we were already familiar and (2) its license permitted its use in commercial software (unlike Jess, which was another contender).
As ScottBurson has already noted elsewhere in this thread, it's not exactly a general purpose programming language. We for example used it as just one component of larger Java-based applications. Its performance, at least for the implementation that we chose, is not great, but there's a performance-flexibility tradeoff: if the alternative is to write-up a bunch of nested if-then statements in Java, and to have to recompile that every time the rules change, I believe that Prolog comes out ahead in terms of development time.