I know I sounds like every other functional programmer but this year I spent the time to learn about monads after 5 years of copying IO examples from the internet. The last few weeks my boss let me do a prototype using haskell and the ease at which it came together was amazing. I don't know if I will ever use Haskell or any other functional language at work, although I can hope, and I don't think that every needs to know how to use a monad but the experience for me of learning a new problem solving tool and then using it effortlessly was extremely rewarding.

I used Haskell at work for the first time about 3 months ago. I reimplemented a chunk of our trading software. It ended up being about 1000 lines of Haskell. Fun :)