Top Gear LOVES Honda's Hydrogen Car, But...
Another problem with hydrogen is getting it to the cars. We already have an electrical grid. It would need many improvements if we were to switch to battery electric cars, but that would be gradual and we could probably handle it because most cars would be charging at night, off peak. There's even a study that found plug-ins might not even require new power plants, and another that shows plug-ins would take less juice than Plasma TVs.
Building hydrogen filling stations is more complex because of the networking effect. It's chicken and egg: People won't buy hydrogen cars if they can't fill them up, and hydrogen filling stations won't be built if nobody is buying hydrogen. Not to mention that hydrogen is somewhat expensive and difficult to transport over long distances because it has a tendency to leak from whatever you try to hold it in.
Conclusion on the FCX Clarity Hydrogen Car
So while fuel cell hydrogen cars are impressive technological marvels and we can dream about a world where most transportation only emits water, we should be realistic about the obstacles that need to be overcome before that world is a reality.
For a long time the auto industry and anti-green elements in various governments have actually slowed down progress in green transportation by fixating on hydrogen as a silver bullet, ignoring other more realistic technologies (I bet GM now wishes it had kept working on the EV1 for all those years, or had started making hybrids at the same time as Toyota).