There are also versions of these motors with full metal gear sets in them. they are slightly more expensive (like $10) But feel a bit more robust. The weak spot on the common ones with plastic gears is where the metal output shaft leaves the motor. It is pressed into a plastic gear and you just have to be careful you don't put too much pressure on it when it is not running because you can twist the shaft right out of the gear. Ask me how I know...... 
