I wrote one once, a bastardized xpath that was used in a web crawler. I think it worked because it was extremely limited in what it could do and because normal code was a fallback in several places, it was never intended for non-developers. With it we could condense several hundred lines into < 10, with much lower error rates. Come to think of it, I'm not sure if the term "DSL" had been coined at the time, or I was unaware of it in any case, "creating a language" was not something I was attempting to do.
Since then I've encountered several in the wild that have been maintenance nightmares. Some were too simple that they were useless. Others were too complicated, so we ended up having to maintain a language and the code in that language.
There's one lying around here that was intended for support people to program in via an expression tree editor with a clunky UI. That was a disaster. We've got another one which is a terribly bastardized regex that was built because "support people don't understand regex". The don't understand this one any better, they would have been better off learning a life skill like regex.
Since then I've encountered several in the wild that have been maintenance nightmares. Some were too simple that they were useless. Others were too complicated, so we ended up having to maintain a language and the code in that language.
There's one lying around here that was intended for support people to program in via an expression tree editor with a clunky UI. That was a disaster. We've got another one which is a terribly bastardized regex that was built because "support people don't understand regex". The don't understand this one any better, they would have been better off learning a life skill like regex.