I personally think that it is an excellent idea. To run off a fuel cell would mean that the aircraft would have an electric engine. They are considerably more efficient (90% conversion rate verses 15% for a gas engine) the engine can produce power beyond it's rate capacity for short periods of time (think take off) and would be quieter than gasoline powered aircraft reducing neighbor complaints at airports. The fuel would be standard hydrogen because there is no need to modify the blend for reducing emissions or because the fuel will be used in an aircraft engine. Electric engines would also mean significantly reduced maintenance costs (fewer moving parts, minimal lubrication needed.
The downside is that fuel cells are expensive as they are are not currently mass produced and there are very few sources of hydrogen right now. And the way hydrogen is currently produced requires more energy to produce than it provides.
I'd love to explore building a fuel cell powered plane. Is anyone currently working on this?