I'm just not really a fan of mixing OO or imperative and FP code in the same code unit, with the exception of a few things like LINQ which might be called FP. For anything with a lot of mutation or tight loops, I'd rather stick with C#, and save myself the hassle of explaining the code to other devs later on. The idea of one language for everything is nice, but C# has carved out a place for itself, and maybe F# should try to focus on those areas where it can differentiate itself by effecting shorter, more self-describing code.
