I have used online check in for every trip since they offered it (except BLT opening, when it was restricted). I can say the one time I was especially glad that I did it was last year when I checked in during one of the big computer down times. At the front desk, they were having to write everything down manually for people who had not done online check in, and those people had to go back to the front desk when the computers did come up to get their KTTW (they just gave out key cards). However, since I had done online check in, my keys had already been issued with the charge privileges on them, so I didn't have to go back to the desk again to get a new KTTW.
So if anyone hears of a big computer shut down for a day or two, and your check in day is affected, I encourage you to do online check in a few days out to make things much easier.