I pulled it out of the drawer again a few days ago. It was lovely, I wrote a monad for asynchronous HTTP results (basically the async monad + an Either monad + some methods to produce some HttpErrors) and encoded a bunch of API functionality (including auth filters) with early returns. The only pain was not having anonymous dynamic objects in C# new { syntax = "syntax" } and using Dapper.

I'd definitely like to give it a go for some bigger projects.