For me personally it is the most productive and expressive language I have worked in.

There is a steep learning curve, which will make one a better programmer, but not without significant buy in. There is no free lunch.

Haskell is very expressive with its types, especially with regard to when effects happen, which makes it excellent as a shared design language. It's interesting for me to see Java/C# programmers struggle to explain some of their more modern stream abstractions to each other:

http://stackoverflow.com/questions/28459498/why-are-java-str...

The answers above are unable to explain succinctly what the APIs are doing, because the authors lack the necessary common language. They have to answer with wordy essays describing various scenarios and use cases.

Not the OP, but I don't think Haskell will ever be drop-in replacement for mainstream langs for cultural reasons, but that doesn't mean that those who do engage w/ it don't get get tremendous value from it or fail to find it their favorite language (I.e. Doesn't contradict what willtim said). And I think that's OK. (No, you're probably not going to convince a bunch of rubyists to use haskell). You can even train entire teams of willing Haskell developers from scratch, if needed. But the desire / buy in has to be there. IMHO anyways

Part of the problem is that dev is so large that it is hard to make "in general" statements anymore.

It's easier to talk about concrete/specific instances or cases