about the only nasty surprise that I can think of is when you discover how hard it is to find two available seats in business class with Aeroplan and Air Canada...

Welcome to Flyertalk! My mother flew HNL-YYZ on AC and YYZ-SFO-HNL on UA in business class and she much preferred the AC flight because it was operated on a int'l 767 (1-2-2 type seating) with nicely reclining seats and all. She also found the Executive J service on AC better than UA's biz service. Plus, AC goes direct, which means fewer US customs hassles and hubbing at any of ORD/SFO/LAX/DEN.

I'm not an expert on this route - I think that there's actually an Airbus 343 that continues on to SYD from YYZ that runs this route too and those seats are very comfortable too.