The good:
Billy Bishop Airport is downtown (alternative is Pearson which is in Mississauga).
Free refreshments in the waiting lounge.
The bad:
Flight timing is unreliable. In the past 3 months alone, I've twice had an EWR to YTZ flight delayed 4+ hours (each airport blaming the other), and twice had flights cancelled altogether (one due to bad weather and one due to an "aircraft issue"). My suspicion is that when they do not have enough seats on a flight sold, they modify the times so that they can consolidate passengers from two different flights onto one flight.
The verdict:
I really wanted to like Porter, and I did at first, but from a scheduling perspective, they are not reliable.