For reasons others mentioned (dead battery, etc) it's likely not a total replacement for a key or fob. However, it could be a great extension or companion to a key or fob. E.G. if you lost your fob, or you want to grant access to a friend, to either drive the car, or retrieve items from the car, that part is cool. Even if the bluetooth raises a security concern (and I doubt current key fobs are much if any better), at least with a smart phone app there is the possibility to update the app and fix security issues unlike key fobs.