I've used Mobile Check-in countless times, stayed at brand-new properties which had the proximity keycards (instead of mag-stripe), but I've never been to a Marriott property where the mobile key function actually worked.

To be honest, unless the front desk has a line rivaling the Hotel Penn in NYC, I'd much rather have a physical keycard. Too many things to go wrong with a phone app (app itself, internet connectivity, battery issues, phone unreliability).