I actually made a brave effort to use Prolog for serious development, and Paul pretty much nailed it on the head: it just does not scale, mostly because its all logic programming all the time. That can make normally direct things rather painful. OTOH, I am using PAIP Prolog embedded in my big CL application and getting huge mileage out of it -- for a tightly defined problem that does indeed benefit from Prolog's strengths. Basically Prolog's mistake (iff it wanted to be an all-porpoise language) was not offering other paradigms.
