Originally Posted by travelingman79

You can still check-in online, and you can still use the united.com bag drop in T1 to deposit your bags.

On return from YUL, you'll pre-clear US immigration and customs prior to going through the security checkpoint.

I have traveled on the first UA (UX actually) flight of the day YUL-ORD once before. The airport was quite busy for so early in the morning, and I appreciated the premier check-in at YUL. Immigration went smoothly and quickly, and only a short delay getting through customs. Do be sure to have your customs form filled out before proceeding to immigration.