Je t'aime, YUL.
I've always had a positive experience flying in and out of here. Getting through immigration upon arrival looks like it's daunting because the lines are so long but they move fast. On the return trip, I've always been through security and immigration (for going back to the US) in 20 minutes or less.
The immigration people are friendly (even when I was wearing some Maple Leaf stuff) and the place is clean and fairly well organized.
Only drawback is if you're flying on a small plane, you have to go to a lower gate area and it seriously smells like jet fuel down there. I was woozy by the time my flight was called. Oh, also, it seems like you have to walk about 8 kilometers from the time you get off the plane until you get to immigration. That irks me, but only because I'm lazy.
Finally, the pickup spot for the 747 Express Bus to downtown is clearly marked and easy to find. I'm a fan.