I'm currently on flight 257 PHL-PHX using the inflight WIFI. I've activated it on both my laptop and my iPhone.
My experience so far, surfing the net is great. Speeds are surprisingly good, but... I haven't been able to stream any video.
As for the iPhone, I've tried installing a few apps, the download speeds are dialup slow, if not worse. That said, using the WIFI in apps like FlightTrack Pro or Flight Update pro is pretty quick. Finally the live map features are useful in flight! We're currently near the Texas/New Mexico border, 30,000 feet up, flying at 496mph and 543 miles from PHX.