Any experiences with Protocol Buffers?
If you are looking for user facing interaction, stick with xml. It has more support, understanding, and general acceptance currently. If it's internal, I would say that protocol buffers are a great idea.
Maybe in a few years as more tools come out to support protocol buffers, then start looking towards that for a public facing api. Until then... JSON?
Protocol buffers are intended to optimize communications between machines. They are really not intended for human interaction. Also, the format is binary, so it could not replace XML in that use case.
I would also recommend JSON as being the most compact text-based format.
From your brief description, it sounds like protocol buffers is not the right fit. The phrase "structured content created by hand in a text editor" pretty much screams for XML.
But if you want efficient, low latency communications with data structures that are not shared outside your organization, binary serialization such as protocol buffers can offer a huge win.