yes you can. C# / F# interop is not zero hassle but it is low hassle. Most of the time it just works. There are a few features in each language that are not zero hassle, if a C# api uses a custom implicit conversion, calling it from F# requires you to explicitly call the implicit conversion (ha!) and if an F# api returns something F# specific like an Option type, using it in c# can be a pain.

At my company we have a C# codebase for a large web application, but we create a 2 stage caching feature for it in F#. This F# project uses a couple of C# oriented libraries (stackexchange.redis and protobuf-net). So lots of interop going both ways.

Some examples of people using it in production: http://fsharp.org/testimonials/