With its round grip, I think the MD-12 is one of the more comfortable motors. It's very easy on the hands, unlike the square grips of other motors. MD-12s are indeed very loud; they make a typewriter-like clacking noise.
As for the OP's question, my MD-12 does react when I press the trigger without the camera attached. Doing this makes the gears jam though--the motor won't react again until you reset the gears by twisting the coupling prongs by hand. The coin trick works too.