I used it for a graduate course in formal semantics. As our final project, we had to implement Lisp in Prolog and prove the semantics formally. Until that point, I thought I had a good grip on Prolog. Debugging the project nearly drove me nuts! If I had a do-over, I'd learn the SWI debugging tools before attempting it.

I love Prolog and hope to spend more time with it some day. But, the step from Novice to Intermediate is steep.