My best success (out of many) was on December 23rd last year YOW-YYZ during the huge snow storm we had. Almost everything was being cancelled. When I woke up, I saw my flight had been cancelled and the hold times were over 2 hours long. Went on twitter and they were able to get me the last seat on one of the 2 flights that left that day within about 10 minutes.
I was really appreciative since the alternative would've had me sitting in a train for 18 hours (my backup plan was the train that got stuck somewhere between Kingston and Toronto).