I have to diverge from the answers provided by "dabeaner" and, less so, with hublim. As with any technology it's expensive at first. Yes, even gasoline was expensive even though it was thought of as a waste product of oil production. A waste product? Yes, oil was originally used for lamps, not cars.
With that said the oil companies knew they had a winner if they kept the prices down and the convenience up. If you think about it cars really didn't "catch on" with Americans until after WWII. Before that much of the population used mass transit and only the upper middle-class and rich could justify car ownership.
Hydrogen fuel manufacturing can be set up right where you buy your gas. They are already doing that, as a test project, in Greenland. The structure, though as large as a store, is nowhere near as large as an oil refinery.
The real problem is storing hydrogen in the car. And the best way to do that right now is with a metal-hydride matrix. More hydrogen can be stored this way than storing the gas under pressure at 5,000 PSI or even as a liquid. The problem is the expense.
I think eventually someone will come up with the metal-hydride matrix that is cheap, easy to manufacture, and easy to mass produce. Once that happens everything else is likely to fall into place....even the production of hydrogen.
As for fuel-cells they are inefficient. They will eventually be more efficient, but right now the more power you draw from one the less efficient it is.
I think the intermediate step will be what Ford is exploring. Hydrogen Internal Combustion Engines. They've found they can build cars that run on hydrogen with a standard engine block, by making some rather minor changes to the engine as a whole. The basic block is unchanged, but valves, injectors, spark plugs, and the lubricant will have to be able to withstand higher temperatures and pressures. These changes add an additional 1/2 to the price of the car making a $20,000 car more like $30,000 to run on hydrogen.
For that the Hydrogen I.C.E. burns cleaner, produces three hundred times less carbon dioxide, four times less nitrous oxide, and no sulfur oxides. And when mass produced that 1/2 will likely come down...way down.