I agree. I recently converted some large files that were previously stored using XmlSerializer to use protobuf-net, and I found an 8x increase in space efficiency, and 6-7x increase in (de)serialization efficiency. It really is a fantastic library, and if your classes are already marked up for serialization, there is very minimal work required to make the switch. For files that need not be human-readable, protobuf is definitely the way to go.