I've used it to write toy calculator grammars, a parser for chemical formulae that calculates molecular masses, and to convert a user-friendly multiple-choice question format into Something Nasty required by an e-learning system (WebCT). It's simple to use, but like others, I found it slow (although precompiling the parser helps). On the other hand, I often find myself thinking, "Oh, I could do that much more elegantly in P::RD," when presented with a problem that I would previously have used nasty regexes on.