I installed a powered attic fan in a house I built about 20 years ago. It wasn't solar. It just had a thermostat that caused it to run anytime the attic temp was above a certain temp. I suspect that it was much more powerful that a fan powered by a 6" solar cell, though.
It certainly made the attic more pleasant in the summer, but I never noticed any drop in my A/C bill -- probably because the house was so well insulated that the attic temp wasn't much of an issue. Or maybe the reflective barrier I installed on the rafters when I built the house was all it needed. In any case, it quit working after about 5 years and I never replaced it. And that's the main problem I see with these things -- moving parts in a harsh environment. I would imagine that the solar version has an even worse lifespan.
One other thing to consider -- if you have a house fire when that fan is running, your house will burn down much faster. They really need to be connected somehow to the smoke detectors, but I have no idea if you can find one like that today. Mine wasn't, but I had a switch so that I could at least turn it off if there was a fire and I had time to do it.
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