This picture shows the results of a speedtest while using the GoGo inflight WifFi on a flight between ATL and MCO (Orlando). I was also able to connect to my Slingbox and view a reasonable quality picture. I have read elswhere though that the WiFi speed can vary as you cross the country, I guess as you get closer or further away from a ground station.
Below is the message, which I found very interesting, letting users know that the WiFi would be shutting off soon.