My bias may be showing in this review; all things considered, Pearson is a 3.5-star facility (mostly because of the uppitty staff), but I love all things related to Toronto/Mississauga so the stars were rounded up.
My main piece of advice (that I'm sure others have mentioned in the 200+ reviews but I'm going to pretend that I was the first to think of it) is that you wear comfortable shoes, because you will be walking. A lot. There are not a lot of moving sidewalks to speak of, but getting to F90 (for example) in Terminal 1 (from the main doors) will take you quite a while. Take into account the long periods of waiting in line, and if you're not careful, the time will get away from you and you'll have to do that odd half run/walk thing that everyone seems to do in airports. At one point, I saw a sign that explained that it would take seven minutes of walking to get to a specific gate. The fact that they need to mention that suggests that many travelers are unaware of how long it takes to get around this airport. The facilities themselves are clean (enough), and doesn't look dated or anything.
Now about the staff: I get that they see a lot of people, and I'm sure they get grief from some real jerks over the course of a day, but I'm not one of those jerks. I do what is asked of me, and I do it quickly. So cut some of the attitude, maybe? What happened to the whole "Canadians are so nice" bit? I didn't get that at all in my (now multiple) interactions with Pearson's staff. Especially the Customs people; you take yourselves way too seriously. I'm sorry I didn't remember the exact date and minute that I was in Canada last time. Cut me some slack, I can't even properly remember what happened in my life last week. The aloofness and general lack of hospitality is countered by their efficiency though, so it's fine I suppose. If they were both rude AND inept (like Philadelphia for example), then that would be particularly frustrating.
Bonus: Free wi-fi. It's speedy enough to help you do what you care most about, which is stalking your ex on Facebook, or... writing reviews on Yelp?