I'm going to assume we're interested in getting off fossil fuels, not just making a half-hearted 10% or 20% reduction in fossil fuel use.
Is hydrogen part of the answer to sustainable energy? Could we power all our transport with hydrogen?
When discussing this question (or indeed any question to do with energy), it's important to do the numbers.
The principal concern about hydrogen is that you need energy to make it -- it's just like charging a rechargeable battery. (And rather a crummy battery, at that - Making hydrogen is only about 50% efficient, and using hydrogen is about 50% efficient, so overall, hydrogen is a 25%-efficient battery.) So where is this energy to come from, sustainably?
But we haven't got 100 spare nuclear reactors.
What about wind? To generate the 42 kWh per day per person, on average, from wind in Britain, we'd need 300 GW of windmills (allowing for a 33% capacity factor, so that the average power delivered would be 100GW). It is possible to put 300 GW of windmills to generate hydrogen. That would be 150 times the current fleet of windmills in Britain; 100 times the current Danish fleet; and 15 times the German fleet. Indeed 300 GW is four times as much windpower as the whole world has today. We could put that much wind-power on Britain. For example, we could turn the windiest 20% of Britain into windfarms. Or we could put them in an offshore windfarm: a strip of sea 10 km wide all around the UK coastline would have the required area.
What's my message?
Talking about hydrogen as a fuel is missing the point. Our current transport habits require energy. Getting sustainable energy is the most difficult thing; using energy more efficiently is essential. Hydrogen is just one (rather inefficient) way of storing energy. Talking about hydrogen doesn't solve the real problem. If we do manage to produce sustainable energy from nuclear or renewable sources, we might use hydrogen, but I wouldn't be surprised if, for most transport purposes, electric cars with rechargeable batteries are a better idea, since hydrogen is only a 25%-efficient battery, whereas electrical batteries are at least 85%-efficient.