I actually had a bad experience this last stay with both in check-in and check-out speed. I waited a long time for the check-in and found out after the fact that my elite mLife status would let me to be serviced faster.
The check-out kiosk did not work for me (told me to go the front desk) and I had to wait in-line for a LONG line - totally wasted my time!
by the way, MGM sent me an email to help me check-out via mobile. The only problem is that I got that email AFTER I landed in Newark. I think MGM's tech people should know that check-out transaction is time-sensitive?! Not very helpful there!