I've used f#. Type providers were really cool. And I like how terse it is but I feel like the gap is narrowing, and I couldn't ever use it on a real project. I'd feel guilty building a project for a client that is supportable by a tenth the number of developers without very good reason.

I'm generally an FP advocate but F# on the CLR has always been an uphill battle in terms of development of the language (especially lately) and providing a compelling case to outsiders to spend resources on F# over C#.