I had never heard of this before so I just looked it up, saw the video of the OCC guys demo the McCuff.
First, where do you store the thing while riding? Do you really want to toss a gasoline smelling hunk of plastic in your panniers or tank bag after filling up?
Second, is this really necessary? I just put the tip of the gas nozzle under the inner ring of the tank (the one with the holes in it) and lean the nozzle towards the pump. It almost stays there by itself but I always hold it to be safe. The nozzle kicks out when the fuel hits that ring. I can give it another squeeze or two, not even a few tenths of a gallon, but after that it will overflow so why bother? I've never had fuel splatter on the tank while filling or when the nozzle kicks off.
The thing seems like a gimmick to me.