solar attic fans - worth it or just hype?

My attic gets ridiculously hot in summer, like you can barely breathe up there. I've been reading about these solar-powered attic fans that supposedly help move air around and cool things down. But I'm kinda skeptical...do they actually make a noticeable difference, or is it just another gimmicky thing? If you've tried one, did you notice your house feeling cooler or your AC bill dropping at all?