The Terravision service has the potential for being a viable competitor to National Express but it seems the timetable is unreliable which is risky when catching a plane, organisation rather chaotic and customer service leaving a lot to be desired. I have used it on two occasions as it very conveniently makes a stop at Redbridge underground station very close to where I live so it is a plus point over National Express. However on an outward journey the coach was late but luckily I'd aimed for an earlier one than I needed for my flight. Coming back from Stansted from a late flight the queue was long and the loading was slow so by the time we got to Redbridge the tubes had stopped running.
If it got its act together and listened to it's customers it could be a useful and reasonably priced service.