Flying out from YYZ can be a hit or miss. US customs can be annoying, being forced to carry your luggage through customs, then to security. They have dealt the slow speeds in US customs by adding machines to fill out the customs forms, thank god. They have also started being smarter at security by allowing people to go to the Nexus line for better lineup control.
International flights are usually pretty smooth for customs and security though there tend to be large families or tour groups at airline checkin that take up a lot of space - they need better crowd control.
The airport lacks decent food establishments. Swiss Chalet and Tim Hortons are the only two reasonably priced options and even then are mediocre.
Public transit to the airport did not really exist until the recent addition of the Union Express which still probably costs more than an UberX. Taking the TTC and transferring at Kipling is not the greatest option, either.
I do love that the airport is very clean and has free wifi. Believe it or not, a handful of American airports do not provide that.