Sounds much like my own experiences. I recently wrote some APIs in C#, and was reminded just how verbose and limited my writing becomes.

Even if I'm writing code that's not very functional, using F# as a better C# seems to work very well.

The main reasons to use C# are for legacy interop and perhaps if the code isn't a major focus and hiring someone capable of writing F# would actually be a limitation. Some folks also feel F# is clunky for imperative code; I haven't really felt that way.