If you belong to an airline club...

I use curbside check in always if I am checking bags on UA. It is very fast and convienent to check bags and then return the rental car. They do not issue boarding passes and I can go straight to the RCC to get boarding passes (and at the RCC you will get to know the check in agents and often get free upgrades and other perks). I believe you should avoid the regular (even Premier lines) at the airport as they see many more people, hear many more complaints and are generally not as nice. Obviously, this works only in cities with a RCC.

And, NO curbside is not the reason my bags get misplaced. That is due to 5 minute connections at O'Hare.