Neat concept,but when I decided to ride again after a 20 year hiatus,I took a rider safety course...grilled into our heads,and rightfully so,BE AWARE of what's around you,constant mirror checks,shoulder checks while changing lanes,and so on. I personally would rather be aware,alert and defensive in my riding,than be complacent that a battery operated device would warn be of trouble from behind. If you buy it,I would most certainly follow your opinion on how it serves you.
My wife's new car has the lights that flash in the side mirrors that warn of a car beside you,my question to her was,why don't you just shoulder check before switching lanes like you were taught in drivers ed when you were 16.I programmed her car so that when I drive it with my key,the flashing lights don't go off,I found them distracting.