[fserb]

Saying that reducing 50% of a 30 bytes message reduces 50% of the client-server exchange traffic is misleading (and wrong).

The average HTTP header is around 700 bytes plus 40 bytes of TCP/IP header. Since its gains diminishes with more data (MsgPack seems to focus on the structure of the message), I imagine the real gain will be between 1-2% of traffic.

The extra complexity is simply not worthy.

[ch0wn]

Also, if you're using it in the browser, JSON is already there while you need to load a library in order to use msgpack.