Funny, i flew an intl->us connection at yyz a week ago and the priority line worked very well. There was a moderate snake of people in the regular queue and only 2-3 people in the priority line.

And they had someone checking boarding passes at the entrance to the security lane too. I had the impression she was actually checking for business class *G boarding passes.

On the other hand I flew a intl->domestic connection at yyz a few weeks ago and there was nobody at the queue.

But I agree that the system described above where the priority queue merges with the regular queue just before the breakout to the individual lanes works best. That also helps avoid the case where you get stuck behind the guy with five million things in his pockets even if he's a frequent flier too.

One thing I wish for though is if they would post a clearer list of who is allowed to use it. It's not like the list is very dynamic at YYZ. I wasn't sure myself if AC*G was enough or a business class ticket was required.