I had the worst customer service experience of my life with Air Canada over the summer. I've never experienced anything like this before.
I was flying United from RDU to YUL with a connection at IAD for a long weekend in Montreal. As I'm boarding the plane at RDU I get the notification that my flight from IAD to YUL is canceled because of weather. Look at the weather map and the storm won't impact things all day, so my thinking is that it's better for me to get to IAD since it will be a lot easier to get on another flight from there as opposed to figuring it out at RDU. I have status with United so I'm able to sort it out with them on the phone before we even take off from RDU. They put me on a United flight from IAD to YYZ and an Air Canada flight from there to YUL and tell me that when I get to Toronto that I just need to talk to an Air Canada person and get my last boarding pass from them. Great. All told it only added like an extra 3 hours to my trip so not bad for having a flight canceled.
Fast forward to when I land in Toronto, I go to an Air Canada desk and they're like we don't have you booked on anything, why were you sent to to Toronto. For some absurd reason, Air Canada does not do any new ticketing at their desks in fucking Toronto, it's only by phone, and I'm told to go wait in this line at a literal phone bank to talk to a CSR. I wait almost an hour in this line plus another hour on hold and the CSR, when I finally get one, tells me that I'm not booked on any flight but that I can buy a $700 one way ticket from YYZ to YUL. I was like fuck that let me speak to a supervisor and got pretty short with them when they refused basically saying "I don't live in Toronto, I have no reason to be here, I can sit on the phone all day until I'm connected with a supervisor". Finally get patched through to a supervisor who then tells me that I missed my flight to Montreal because they could all of a sudden find the booking so it's not their problem.
At this point I'm like 4 hours into dealing with Air Canada's bullshit, so I go back over to the United Premier desk and explain what's going on. They were beyond annoyed and started bitching immediately about how big of a mess Air Canada is and how they pull this shit all the time. The lady at the United desk is like "if you're fed up with Air Canada I can throw you in a first class seat on one of our flights first thing in the morning, put you up in a hotel tonight, and give you $100 for food and other incidentals", so of course I jumped at this because it ended my Air Canada nightmare.
What took over 4 hours with Air Canada for no resolution United resolved in 15 mins. I'll never book a flight with Air Canada regardless of how much cheaper it might be.
TL;DR never fly Air Canada