+1 on this, though I would add that on most long flights there are significant periods where the wifi doesn't work. On a transcon I would assume that it will be non-functional for an hour or so of the trip, spread out intermittently throughout the flight. If the wifi isn't working when you get on the plane (before pushing back), in my experience there is a 50%+ chance it will be out for the whole flight so plan accordingly.

Speed is also variable from flight to flight. Sometimes it is quite usable, at other times very slow.