I've been testing gRPC a bit. It's promising, but the tools are clearly alpha quality, so if you're going to build apps on top of it, you're in for some early-adopter hurt.

For example, you need the latest Protobuf 3.0 beta release, which no current OS distro currently has, and build gRPC from source. The language-specific packages (Ruby, Node, etc.) have releases that lag behind gRPC itself, so you'll probably have to build those from source, too (linking directly to HEAD from a Gemfile or package.json doesn't work, last I checked). Performance also seems decidedly lackluster, though it's been a few months since I did some casual benchmarking.

As for the API and feel of the library, it's similar to Thrift, and much like CORBA without the OO and attempts at location transparency. With the current generators, you'll get some low-level and not very friendly wrappers generated from the Protobuf declarations that don't attempt to hide the fact that you're writing RPC requests and responses as Protobuf structs.

One disappointing aspect of gRPC at this point is the lack of discoverability. Clients have to connect to a specific host and port, and you have to build your own glue based on ZooKeeper/etcd/Consul/DNS/whatever. Since no fault tolerance is built in, things like retries and load balancing is left as an exercise for the reader.