This app is highly regarded on flyertalk, but the issue is that airlines stick to their own rules.
For example, flighty will tell you that your flight will be delayed by two hours as the incoming aircraft that will serve your flight is 2 hours late leaving wherever it's coming from.
So you get to the airport later than planned, so as not to waste your time. Except the airline has closed the check-in desks and you miss your flight.
Or your airline decides not to wait and uses one of their spare aircraft to run your flight on time. Flightly works this out, but doesn't give you enough time to get to the airport.
As for the price, my understanding is that they pay for their data feed and it's not cheap.