I use the contact us feature at the bottom of the aircanada.com page after about half of my flights.
I'd say it's an even split between great service (ie Bruno from LGA to YYZ this week) versus awful service (last month coming back from HND). I want to give feedback so Air Canada knows who are the good staff and who are the bad.
Also, I usually wear comfortable clothes while flying, so when I'm in business class in my loose jeans or hiking pants with a shirt and hoody, I get treated differently. Sometimes I get better service (they think it's my first time in the front) and other times they ignore me and think "look at that punk/slob/trash" because I am not in a suit and tie, especially coming out of LGA.
tl;dr leave feedback for both good and bad staff so that Air Canada can keep track of their customer service