One advantage of being reasonably proficient with Haskell is being able to very quickly create maintainable programs, often without needing to write extensive documentation.

One disadvantage of having Haskell as a favourite programming language is sometimes looking like a pretentious ass.
But just read the type definition and think for once, idiot. How could "Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r" confuse even a beginner to computer programming, nonetheless Haskell?