For me (in So. Cal), my morning commute is always cool (low to mid 70's). It's warm at lunch, but the trip's so short... and my office is freezing, so it feels good! For the evening commute, it's not too bad since I'm leaving at 5:30 or 6 PM. Temps on a "hot" day are mid 80's at the office and mid-90's at home. Dry enough that it's not too bad. On the extremely hot days, I do use the A/C.

Having lived in New York, Houston and Baltimore in my past, I know how nasty that humidity thing is. I remember once having to use my wipers on a clear evening in Houston... the humidity was so high that moisture was condensing on the windshield (cooled by the A/C inside)!

 

Simple Aerodynamics. With the windows down on any vehicle, the drag coefficient is increased and therefore the engine must work harder to keep the vehicle at normal speeds. This effect increases the faster one is travelling. I believe the AC compressor uses less energy than rolling down the windows. Yes, if you have the AC blasting so it's arctic in the cab when it's sweltering outside, there will be a serious hit to the MPG. However, keeping the AC at a comfortable temp. (quite easy with Automatic Climate Control) you will actually see less of a hit on MPG than if you were cruizing down the interstate with your windows down, and still sweating.

 

I never drive at freeway speeds with the windows down for just that reason. My windows are open just a crack... enough for draw some breeze, but no impact on aerodynamics. Another reason why I use this method... wind noise with windows wide open is annoying!