Face recognition based actions

I’ve seen quite a few apps have a face detection smile to confirm or submit feature lately. This usually comes along with a button to do the action if you can’t or don’t want to smile or if the user just wants to get on with it. I am wondering if this is good UI UX and if there are any other people around here who’s had experience with designing or developing this feature. So my question is, will it give good user experience and will it be worth implementing.