I'm not sure about the "learn it as a first language" bit.
My university taught Haskell as its intro language to math and science students, and the people who had the most problems where those who already knew a bit of programming. People for whom Haskell genuinely was their first introduction to a programming language on the whole had no problem getting it. I imagine this might be the same.
The main downside with the approach was that when these people who had just learnt Haskell moved on to the next programming course, that was taught using Java, they had a really hard time understanding what was going on.
My university taught Haskell as its intro language to math and science students, and the people who had the most problems where those who already knew a bit of programming. People for whom Haskell genuinely was their first introduction to a programming language on the whole had no problem getting it. I imagine this might be the same.
The main downside with the approach was that when these people who had just learnt Haskell moved on to the next programming course, that was taught using Java, they had a really hard time understanding what was going on.