Every few months I try to learn me some more F#. It isn’t easy.
I stumbled across a nice learning tool by Chris Marinos. It works on the premise that you learn by making the unit tests pass (initially, they all fail). It's a nice idea. The only issue is that the testing framework it uses was written in NUnit. I don't use NUnit anymore. With a couple of little changes I switched it to Microsoft's unit testing framework. You can get my version here.
There is also http://www.tryfsharp.org/. It gives a good intro to F# that you can do online (don't need Visual Studio or nothink).
I tried to do some interoperability between C# and F#. When testing an F# console app in a C# test project, my functions didn't return results. I changed my F# console app to a F# library and it worked correctly. (Don't know why C# can't use an F# console app.)