.
I got one of those given to me and thought it was a waste of time. Figured I should at least try it though... Turns out you do have a little better control with the handle on the can - like a real paint gun. Mine didn't have a special nozzle though. Just a can holder. You supply the can and nozzle.
I recently painted a 3ft x 2ft box with it on the back right of my wrecker. It looks incredibly smooth and even and I used the can gripper.
That information and a nickel won't but you a cup of coffee though