Hi,
The best that can be said about it is that it probably uses as much energy as that used to produce the energy to power an electric car. It's still dirty in the same way as an electric power station is dirty.
http://en.wikipedia.org/wiki/Hydrogen_production
Nothing is free. To produce the energy required to run a bus is more than the actual energy expended in running the bus. Most buses have great big engines, so a V16 expenditure to produce the energy to run the bus is not unreasonable at 80% efficiency at best in producing the energy in the first place.
Paul