I've found Flight+ (now Flight Tracker) to be pretty good. I've used it to track about 15 segments so far. On Friday, it alerted me to United delays about 10 minutes before the delay appeared on the airport displays.

My biggest beef is that notifications are not reflected back into the actual flight. So if a flight is scheduled to depart at 9pm, but then you get notifications saying "Now 9:30", "Now 9:45" and then you look back at the fight info itself, it still shows 9pm departure time. Maybe that was a one off.