I have been writing Haskell professionally for about 4 years now, and I absolutely love it. I'm constantly finding myself having to answer questions of "So, what do you mean?" or "what is this" so early on in development. Never before have I had a language that is so directly tied to my understanding of problems.

On the practical side, I love having types that let me refactor so mercilessly, along with a very mature compiler with a decent suite of warnings.