The most notable advantages are:
Items are received very fast and in the order they are sent.
items are received more reliably according to some tests i performed quite a while back.
Testing C2DM:
server side, I sent messages from a loop (php) to my device. The device displayed and logged the messages received. The log showed some quantity of missed messages (some were not in the correct order as well) every time the app was started an an let run for a while.
Testing Pubnub:
server side, I created channels in a loop. On each channel i sent some quantity of msgs and logged them.
In my web browser i could view the list of channels currently available and clicking on them displayed the messages sent. All messages were received correctly and without having to refresh the page.
This was quite a while ago. I'd have to do some digging to set it up again if needed.
Chris
More info. I posted a new thread with some information about pubnubs services. I hope I put it in the right place. I'm very eager to get this msg'ing system going in my app.