first and foremost @Sean Timarco Baggaley: "And no, you don't get to whine about The System: either get *involved*, start your own party, or shut the f*ck up. If you're not willing to fight for it, why the hell should others fight on your behalf?"
@Torben Mogensen: "This is, indeed, a problem with .NET languages: They are all tied so strongly to the platform that they are of little or no interest outside this" & "...strong adoption of a functional language that is not tied to a specific platform"
IME the biggest gains is the paradigm. You need the language to support them but the underlying ideas are where the real value lies. Once you've 'got' FP, there are plenty more you can get unlinked to windows.
@ratfox: you're right, I've seen some horrible abuses of FP, but there is an underlying smoothness too.
@Marco van de Voort: you can (if you're capable) wrap up more functionality with functional stuff so simplifying conceptually and reducing lines - both reduce bugs and speed development. This is from experience.
@jlocke: FPs are *models* of computation; you specify what but that doesn't mean they have to do it that way - "The reason for this inefficiency is tightly coupled with the advabtages of functional programming - the lack of memory modifications" - Modifications are certainly permitted in the implementation if they do not modify the semantics of your program. MODEL != IMPLEMENTATION, always. Performance still is an issue though, I grant (IME).
@Colin Barfoot: "Somewhat ironically Haskell was a classic software failure: not meeting requirements." - eh? what requirements were these? AFAIK it was an experimental lang for trying stuff out. As such it seems to have succeeded beautifully.
Further: "Declarative and procedural languages do not mix" - if by declarative you mean functional then that's rubbish. I've mixed them easily and confortably (when the facilities are supported by the language) and can switch back and forth easily - and I'm no FP expert.
I'm *no* FP expert but I do like it and would get excited. But it's MS, a platform I've found I want to abandon.