I use Tripcase and I am happy with the alerts. In an airport I always got Tripcase alert about a change of gate or a delay - before it was displayed on an electronic board or announced.

I once got a Tripcase delay alert for 10 or 15 minutes. It was not displayed or announced. I asked the agent at the gate about it and she said: This is correct, but we do not tell people about such small delays.

Tripit also has flight alerts, but only in the Pro level that costs $49/year. However, many companies offer it to their employees.