Hydrogen cars are widely considered to be the ultimate replacement fossil-fuel-burning vehicles. Advantages of hydrogen, according to Keith and Farrell (2003) are (1) it can be burned cleanly or used in fuel cells and so can reduce air pollution; (b) it emits no CO2 at point of use; and (3) it can be produced from diverse energy sources and so can reduce oil dependence. However these authors also point out the disadvantages: (1) it is estimated that the refueling infrastructure would cost abut $5,000 per vehicle; (2) the cars themselves will be more expensive than current cars; and (3) although hydrogen is simple to produce, it is very expensive to transport, store and deliver all of which are strong negatives for its use as a transportation fuel.
Reference
Keith, D. W., and A. E. Farrell, 2003: Rethinking hydrogen cars. Science 301, 315-316