I just accepted a small consulting job that is to be written in Prolog. Ten or fifteen years ago, I was pretty much a Prolog guru (I once rewrote a planning system that had taken 6 weeks to write in Common Lisp to a Prolog version - took just 4 or 5 days -- and the Prolog version had better graphics than the Common Lisp version).
Fortunately, I worked through the excellent book Art of Prolog a few years ago, so I am not that rusty at Prolog programming. When Prolog is an appropriate for a problem domain, probably no other programming language is as productive. Good applications of Prolog include search, circuit layout, planning, and natural language processing (but not statistical natural language processing).