Personally I think they're great for development, but less great for learning. The advantage with something like Haskell is that it forces you do to things the 'right way' every single time and puts all the functional concepts right at the forefront. Once you've internalized the concepts then you can easily go onto other languages and bring those concepts with you.

While I haven't really written anything in Haskell since leaving school, learning Haskell has informed (and in my opinion improved) the way I program in every language more than basically anything else I've done.