I am evaluating Google's Protocol Buffers for my knowledgebooks.com KB_bundle product
While I designed KB_bundle as an embedded Java library, I have always planned for both RESTful and SOAP web service support. I have been looking at Google's Protocol Buffer documentation and examples this weekend and I think that I will also supply a third wrapper for Protocol Buffer RPC support.
Earlier this year, a project that I was working on had performance problems due to the overhead of serializing data to XML and then parsing it in a REST based system. The problem was that when the project started, relatively little data was transferred between back end processes and a front end Rails application so the overhead of using XML was OK. As the project requirements changed, we passed much more data encoded in XML. I am looking at Protocol Buffer in general as a way to avoid performance problems in the future.