Binary format makes me believe it's not human-readable. How doesn't this compare in size to gzipped JSON? JSON overhead is fairly small (some quotes, colons, brackets and keys) - it's no XML.
Regarding GZipped JSON, it is true that GZiped JSON is small. But, due to the CRIME and BREACH attacks it is not recommended to compress data sent over encrypted connections (TLS).
If you look at our performance benchmarks page you can see a list of serialized length comparisons. As you can see, as soon as you send a few objects in an ION table, the difference is big. More than what you normally can gain with GZip (except perhaps for String).http://tutorials.jenkov.com/iap/ion-performance-benchmarks.h...
Furthermore, GZip only helps with transfer time, and actually slows down parsing time. If you look at our performance benchmarks you will see that ION parsing time is a lot faster than JSON. Additionally, if you really, really want high speed you do not parse ION (or JSON) into Java objects. You process the data directly in its binary form. If you look at our read-and-use benchmark you can see just how big a speed difference that gives. ION is designed for being processed directly. JSON isn't as good for that purpose.
Finally, ION is designed for fast arbitrary hierarchical navigation. JSON is not.
Of course, anything more than a simple construct you're better off using a decoder (e.g. a Wireshark one).
Also, the fact that it's compatible with JSON means that you can use JSON in your development, and then switch to CBOR at the end for the reduction in packet size. In python it's as simple as changing:
import json as encoder
import cbor as encoder