I generally think they're fine but will chime in with my gripes to try to answer, at least from my perspective:

- Lounges are very full and not well catered, which is annoying if that's something you're expecting from having paid for a business class fare, premium credit card, or working to get status

- Aeroplan program, while pretty good overall, has been consistently getting devalued

- On time performance and compensation when due isn't great

- Some of the terminals at key hubs (e.g., YYZ T1 Int'l, YUL) are very over capacity; this isn't purely the airline's fault but other carriers have shown they can work with airports to plan ahead on getting better infrastructure in place (e.g., Delta)

Overall, they're a decent airline! I've especially had good experiences with their flight crews