This is a well-organized, clean airport. The staff is (mostly) helpful and pleasant. As far as atmosphere, I appreciate how sunny and open it feels. Some airports feel claustrophobic and damp, but not Pearson.
The only complaint is how bloody huge it is! Sometimes after a flight I feel like I've run a half marathon before I even get to customs. It must feel far worse for people toting kids around, too.
I fly back and forth to the US a lot and I feel like I know the US concourse really well. I'd even say it feels like home. Especially my corner both at the Molson Pub (which is another story...). Happy flying!