Here's my experience with MUMPS. About twelve years ago, I interviewed with a shop that used it. One of the interviewers told me, a little smugly, that MUMPs was so powerful, that nobody can be productive in it for at least a year. Then I got a one paragraph description of it and was asked to write some basic programs, using pen and paper, with someone watching. My recollection was it was a bastard child of assembly and LISP. There were lots of parentheses, but only register variables to work with. (That doesn't seem to match the wikipedia description, but it's the description I got at the time.)
The place probably didn't have a choice about using it, but they seemed convinced it was the best thing ever. I'm not too broken up over not getting that job.