So I've taken Air Canada twice - to Toronto & Montreal from LaGuardia and I haven't had any real problems. My flights have been on time and they're less than 1.5 hours. After reading the other reviews, I apparently should keep it that way. Anything longer, I'll stick with United.
The one problem I did encounter was a lack of personnel at the check in and bag drop counters at LaGuardia. Going to Montreal, most of the check in kiosks were down, the line was ridiculously long and I almost missed the cutoff for my flight. When I came back from Toronto, the process was all over the place - one line to check in, another line to print your luggage tag and another line to bag drop. Huh?!?!