The air wake up is actually nice.Better than double tap function for me.Double tapping your screen don't always work and it frustrate you at times.The air wake up function works flawlessly by just  hover your hand right near the screen and it will wake up. No need to place the phone on a hard surface as the instruction said so.
It works on any position Dont worry about the battery consumption,like double tap it also requires sensor to stay awake. Very handy if you need to check for any msgs with your note 4 on the table and your hands are dirty/oily at the moment