The fact that Rider is using FCS for #fsharp support means that we shouldn't expect much better quality and more features than what exists in currently existing tooling (Ionide / XS). Tho, it will be probably more stable.
Actually I expect the support to be pretty minimal - they don't really want to invest into F# ecosystem, but just support mix language solutions. Open sourcing it is also interesting - someone is hoping that Community will build more advanced support, I guess