I got to fly Porter this weekend for the first time (Toronto to Newark) and returned on Air Canada, so I have any easy time comparing the two.
Porter was good, but I'll be honest, the AC flight was much nicer :)
Porter does have some perks - the downtown location is a time saver for those who already live downtown. Also saves the usual $50 cab fare to YYZ if you don't have a ride to the airport.
The free treats in the departure lounge are nice but not really a big deal.
AC has nicer planes, seats are way less ghetto and I had my own mini-tv on the seat in front of me. Foodage was better. I realize this doesn't matter a great deal for such a short flight, but my AC flying experience was more pleasant, plus I actually booked AC back because the flight was cheaper than flying back on Porter.