I'm simply using cdc.py to print out changes for a particular table, and unfortunately specifying nothing (or JSON) produces one line, and then nothing whatsoever (but the CPU is pegged at 100% for the process).
/usr/bin/cdc.py -u cdc_user -ppassword -h 127.0.0.1 -P 4001 -f JSON ebth_production.items
If I switch the format to AVRO with the same command, it does print out information quite frequently (but it is binary, so not really useful).
It is worth noting that this particular table (items) has a LOT of changes constantly. If I choose another table (like users) it does print changes in JSON properly, but users doesn't have as many changes as items.
Let me know what information you need to further debug.