Be advised - if you use Aeroplan miles to book a United flight, you do NOT get your UA status benefits until you check in. Yes, that means that if you book an Economy Plus seat, they will reseat you in Economy Minus. I was told it's because United has no way of knowing that you're a United status passenger - you're simply Aeroplan luggage until you check in.

Luckily I've never had problems getting rebooked back into Economy Plus at checkin, but I've also never tried to use Aeroplan miles on a really busy flight, either.