I also feel that I wouldn't want to write the whole app in F#, because I don't feel like the OO/FP approach feels as natural for some parts of the app as plain old C#.
Having to explain the Visitor pattern though to someone else would be much more painful than that I imagine versus just writing and explaining it in F#. The code just reads much more naturally for it, since the language supports it without a quirky design pattern.
For curiosity, what in particular do you find to be unnatural in F#?
Yes. It's even possible in some cases to use F# source as some kind of pseudo-code with C#-programmers.