As someone who has flown porter 10+ times....
DO NOT FLY DURING WINTER. DO NOT FLY BETWEEN MONTREAL/TORONTO OR TORONTO BOSTON.
This is the third time they've had severe problems reacting to the weather. They simply are not capable of coordinating customers. Cancellations are incredibly frequent and their policy forbids giving anything to appease customers for the inconvenience. Flying short distances between Toronto/Boston, Montreal/Toronto is not worth it until they can get their act together. Also, staff tend to get rude when things go awry.
I strongly recommend VIA rail or Megabus as an alternative. At least you know you will know whether or not you will make it to your destination and plus it'll be cheaper.
Also, I think they delete negative reviews so that's why the average is high right now.