Well it certainly will save you some cash; how hot does it get where you are? Do you have another roof air that is going to stay? If so, it seems that it may be counter productive to run them both at the same time (whether the Fantastic Fan is blowing in or out).
The Mfg of the fans stands behind their product, and they are well built. I will have two fantastic fans - no AC at all. AC could be added in the future if necessary, but not planned for the beginning.