I did call and specifically ask that if I checked in mobile but needed to cancel, could I still do so. The answer was yes, there procedures to cancel would still apply and I could cancel before 6pm even though I had "checked in".

As for the success of using it, I admit I've had pretty good luck. Sometimes you do have to wait if there is only one clerk and they are on the phone or helping someone else. More often I've found that I just show and give my name and tell them I've used mobile check in, and they hand me my room key and off I go. It usually saves me some time so I continue to use it.