i'd heavily used haskell for 5 years. i was taught by some of the people who developed the original haskell specification, and the people who worked on haskell prime. i aced their haskell course (~95%. first in the year, iirc).

i've used haskell in work i was paid for, i've used haskell for real-time low-level video processing, as well as high level machine learning. i've contributed to a few open source haskell projects. the entire code for one of my research projects was written in haskell, coming to 2.5k lines of code.

you'll often find people who advocate haskell have the attitude that you "just don't get it" or "don't have enough experience" (an attitude you might find purveys some of the responses to my comment... i certainly do). i do like haskell, it's beautiful in many ways, but for large, hard problems where it's tricky to know how to develop code, i found it inappropriate. where you know what you want to write (i.e., have a specification that is solid or are willing to spend a while properly engineering things), haskell is great.

Would you then recommend prototyping in a dynamically typed language, then re-implementing that in Haskell once you know more about the solution space? Exploration first, hardening second?