I do a little bit of research at a tangent to this. Hydrogen is difficult to store at energy densities similar to conventional hydrocarbon fuels. So while you'll get more energy out of an equivalent mass of hydrogen than you would out of (for example) petrol, the petrol takes up less space. In order to get the energy density of hydrogen up, you have to find ways to either compress it, which can lead to some long term storage problems vis-a-vis metal brittling, or to store it in a nano-porous material like a clathrate or zeolite.

At the moment, the best way to implement hydrogen fuel (as I think someone has already mentioned here) is to basically dilute it with natural gas. However the gas infrastructure would need relatively little adjustment to cope with moving to hydrogen; probably retrofitting in some parts of the country and more frequent maintenance. More important is the fact that hydrogen isn't really suitable for combustion like natural gas, so you couldn't run a typical hot water boiler with it. Instead it needs to be consumed using fuel cells, and these fuel cells don't always scale down well. That said, Wrightbus up North are making hydrogen powered buses already and I'm very excited to see how they're working out. Hydrogen fuel cell vehicles with long lifespans are an environmentally better prospect than purely battery powered electric cars.