We have the type LeRoy suggested. Wore them last 4th of July weekend on a trip fromLas Vegas to Reno and back. Temps were around 110 degrees and the vests kept us reasonably cool. With temps that high, there is nothing that will kee p you completely cool. They are great while riding, but get a little heavy and hot when stopped. Overall, glad we had them. Some of the others that rode with us did not have them but have since purchased their based upon how cool we stayed on the ride.
Same as most of the rest, works best with an open weave blow thru jacket. Carol uses the Sahara vest inside a Joe Rocket Phoenix jacket and stays comfortable in up to 100 degree heat (comfortable in this case means bearable - were from the coast where comfortable is 70 degrees and not raining). Anyway, works for her.