I am in Charlotte NC right now at the Marriott City Center and tried the new Mobile Key which is now available for some locations. Just wanted to say I think it is great ! I still obtained a hard key but I left it at the gym by mistake and the Mobile Key worked perfectly ! If you get the chance, I highly recommend using the Mobil Key. Simple download after the check-in clerk sends you a link. Never have to worry about forgetting your key again. I hope this is rolled out to all hotels in the future. Once you have it installed, you simply press unlock and hold it up against your door lock (or the lock to the gym) and it opens. Great new service !