They work just fine at 18V, but you will wear out the brushes/commutator much faster than normal. The company I work at used them to drive our solar robots because I was familiar with them and the VersaPlanetaries. They averaged around 40 hours of continuous operation... we are now using geared motors that are $400 dollars each. I would advise you to put heat-sinks on them, should help prevent them from cooking as easily.
http://hobbyking.com/hobbyking/store...ize_motor.html