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.

My opinion is that it's a double-edged sword. It's like when someone recommends you to learn SQL and relational algebras, and you, coming from OOP world, say, oh, I can learn some ORM first, that will make it easier.

Personally, I found that diving directly into the language that represents a paradigm in the "pure" form (for FP I would recommend Haskell) works best for me. It makes it most obvious why things are done certain way.

A great way to get a taste of FP, but as others noted, not the best way to really learn FP (Haskellbook and SICP are great.)