It definitely keeps the heat down but do NOT use it in a street car. You will ruin your headers. They will either rust really fast or you will burn a hole through them.
I agree with the crowd. My headers only lasted 18 months with heat wrap. It did lower the under hood temp but not worth the frustration and cost of new headers. I would get ceramic coated or a bad ass radiator instead. you will spend more up front but less in the long run.