Just did this flight a few weeks ago (in the other direction, KEF-YHZ), and the plane was full, so a popular route. Continued on to YYZ on an AC flight, but the ticket was issued by Icelandair. Much to my surprise, we still earned Aeroplan points on the YHZ-YYZ portion, and even received an op-up (yay cold chicken plate!).

Icelandair was fantastic, and the country is stunning.