I'm sure the app is wonderful. I've gotten pretty good at finding this data from other sources, though, and one huge problem is that a delay isn't a delay until the airline says it is. If you carry on every bag and have no special requirements, and you checked in online ahead of time (so you have your boarding pass), it's very useful info and I could see paying for the app.

But if, say, you are traveling with a pet that has to be verified at the counter, or you need to check a bag, the time windows for accepting those are set by the scheduled departure time. If your plane is still in the air or hasn't even left its origination airport (and, for the sake of argument here, we will assume you are flying from a smaller airport that doesn't have other aircraft that can easily be reassigned to your flight, so you know it will be delayed), it doesn't matter: they still close the check-in and baggage 45 minutes (on American; YMMV by airline) before scheduled departure. So you have no choice but to get there early and wait unless your airline actually declares the flight delayed when they know it will happen.