Trying to use a phone with the sensor on the back just showed me how often I'm unlocking my phone while it's laying on a surface or how I always pinch the phone out of my pocket by the chin because that's all the room I have with pants that fit. Also, I can't access it when it's mounted on my bike which is such a serious showstopper I don't know how it made it out of the brainstorming phase.

I found it to be the worst place to put it.