I've been using gRPC for the last several months. It's a reasonably nice API and integrates nicely in with protobuf, which I think is it's main draw. We were already passing data around as protobufs, so we just took the dive.

Unfortunately, gRPC is still very much at the release level of "it works at Google and also on some developers laptop once", so while we haven't really encountered any bugs, dealing with gRPC has been a bit of a devops headache. That being said, we don't have a full-time devops person or anything, and I bet someone who knew their way around automated configuration and deployment better than myself could handle gRPC fine.