I'm curions about this too. Haskell is my hobby language right now, I'm still an uber-noob, but my impression is Haskell code is easily 10-15x more robust, but not necessarily more productive than cool-kid languages (Ruby, Python, Javascript).

The robustness of Haskell is one of the things that improves productivity; less time is spent hunting bugs.

Depends on what you are doing of course. But having an impedance mismatch between the language and the problem domain can make simple problems extraordinarily complex.

My own experience. Haskell's just fantastic that way. The type system, the libraries, and the tools really make you productive.