Hydrogen fuel cells are basically a highly efficient battery. They are therefore only as clean and efficient as the electricity used to produce the hydrogen is. That said, even a coal fired powerplant gets way better efficiency than an internal combustion engine. If you move to zero emission generation technologies like nuclear, wind, solar, or tidal, then you've got a truly zero emission system.
I seem to recall that Honda at one point wanted to market this car with a home hydrogen generator that would double as a home fuel cell, to allow you to generate hydrogen when electricity is cheap and then use hydrogen to reduce your electricity use when it's more expensive, in addition to supplying hydrogen to the car.
Hydrogen storage is much much safer than it ever was. We're more than 70 years past the Hindenburg disaster, and the biggest thing we've learned? Don't store hydrogen in a highly flammable container. That's right, the Hindenburg was covered with a highly flammable fabric. There are now hundreds of fuel cell buses on the road across North America and to my knowledge not a single one of them has had a hydrogen explosion. Why in God's name would you want to transport hydrogen when you can generate it on demand from water and electricity. This is the approach taken by the hydrogen filling stations in Iceland. We're actually pretty good at transporting electricity and water.
Brilliant plan. I hope they manage to pull it off. I'd be willing to have an around town car that I could fill at home, or even a neighbor's house, eventually.