They are on the roof midway from gutters and peak. That was one of the conditions to buy the property from contractor. Electrician said smart move because it moves hot air away from ceilings. He used a infrared thermometer on one house he installed fan three years before ours and saw a 10 degree lower temp on ceiling (even with insulation).
As described in other posts. Yes they work. I installed on a small shed I build. It helps eliminate ice dam buildup and internal condensation due to temperature variation if the structure is sealed too tightly.