I just used this this morning. For a ticket bought online at the AC site, it worked fine. Then I tried checking for an Aeroplan reward, and that did not work.

However, I was able to check it on the Aeroplan site. Go figure.

By the way, both reservations had been changed (over the phone) as a result of a flight cancellation, and both came up as changed, with no problem. The Aeroplan site had seat assignments, the AC did not.