We used Orleans for our service framework for Breach, a 4v1 asymmetric action RPG built with Unreal Engine 4. While all the real-time simulation was implemented authoritatively as part of an Unreal Engine 4 server, we used Orleans/SignalR for the service backend. Services included: matchmaking, (voice)chat, character/account persistence, quests, microtransactions, customer service, and friends among others.
It's a great framework and I hope others adopt it for backend services. My only complaint, and it isn't particular to Orleans, is that there isn't a lot of examples that demonstrate best practices for building large applications with it. There is definitely a "right" and wrong way to build with Orleans and it can take you a while, and a lot of refactoring, to discover that.