Fully powered hydrogen cars use only hydrogen for fuel. If you can find a hydrogen station, then having a hydrogen powered car can be very beneficial. But at this point, hydrogen cars have a long ways to go before they get adopted to the growing demand of vehicles.
Like gas powered vehicles, hydrogen cars utilize fuel, the use of hydrogen. The mileage is very discrete, it's estimated that you can go about a month driving in a hydrogen car without having to "re-fill" your tank. While solely powered hydrogen cars are still being engineered, it's now possible to modify your engine and convert your car to burn water, very similar to hydrogen powered cars. The difference is that you can convert your car to burn water today, as opposed to hydrogen cars which is supposed to launch after 2010.
When hydrogen cars become mass produced, the price is going to be steep. If you want to experience what the hydrogen-powered vehicles are going to offer in the future today, consider converting your current vehicle to use water for gas. It's very easy and fun to do. Many people have reported gains of over 100% gas mileage while most report gains of about 35%.
These are of course considering all types of driving conditions. So while hydrogen cars are still being drawn in the drawing board, you have the opportunity to convert your car to burn water. Hydrogen cars are going to double your gas mileage, but at the same time, the price is going to be twice as much, very important to know.