Flew on a Fly-fi enabled aircraft (sorry, didn't get the tail #) on BOS-TPA tonight. Service was great - when it worked! We had it for about half the flight, starting about a half hour after takeoff. The FAs said they'd been having issues most of the day, not only with Fly-fi but with the entertainment system - not sure if they're linked or not.

As mentioned upthread, there is a free level of service, and the eye-watering $9/hr level, which is supposed to let you do Netflix etc, so they must be confident of substantial bandwidth for that price. The freebie tier (or freebie-for-now) was every bit as fast as Gogo, which I use on USAir, if not faster.