I'm a professional programmer and I know Haskell, but I've only ever written one real Haskell program (an AI for double-move chess). Nevertheless I recommend it. All I can tell you is that if you master it -- I mean really master it, not learn to write Python in Haskell -- then your Python programming will reach a new level as well. You will be able to solve problems that once seemed intractable, which you'd persuade your product manager to scope out.

It used to be that you could get this effect by learning Lisp, but I don't think that works anymore; too many of Lisp's good ideas have since been taken up by more ordinary languages.