Both coming into YYZ and leaving I got in and out of customs fast - so I'm going to give 4 stars mostly based on that. The airport seems huge and I've barely been to any of it (2 gates total, so that's like 1/100th of the airport).
The Air Canada folks were helpful and it was easy for me to figure out where I was supposed to be going. Free Wifi is nice, and the security line moved fast. I'd have a hard time giving an airport 5 stars, but I can't see much to complain about here.