well, using magnets for say, something non stationary like a car would be a very big feat. To convert magnetism to energy would take a large magnet depending on the weight of said vehicle. Now, the problem then arises that you must create a bigger magnet to do this, of course increasing the weight of the vehicle and ultimately making you need a larger magnet. So, its kind of a cycle there. You can strengthen a magnet with a constant current, but then you again have the problem of a weight ratio for motion. Usually they are stationary objects that are used for things such as large power stations.