I think there are some inherent problems with solar panels in general:
It may be difficult to keep it properly oriented towards the sun while riding
It may not produce sufficient power in less than full sun
It may be difficult to justify the weight when compared with a few extra batteries
One advantage of the Flex5 is that it can charge anything that can run off of a car cigarette lighter ie: PDA, phone, battery charger. I'll be trying this in the fall.