For a long time I'd only turn on the vent and let the window about halfway down, and hardly ever ran the air to save gas. When I was getting shotty gas mileage anyway, I finally said f@ck it, and have since been running the air. I don't run it in town, and I wait til I'm crusing up to about 55 mph. before turning it on.
So far, I can't tell that it's lowering my mileage any. I haven't filled up a tank of gas and seen how many miles I can get out of it yet, but so far I can't tell a difference in the rate in which the gas meter falls.

I run it all day everyday in stop and go chicago surburb traffic. I get terrible gas mileage so I dont notice the difference. It does lower it but its worth the few extra bucks a month to enjoy the ride rather than sweating it out knowing I am saving a few dollars. It has not changed they way I budget my expenses.

in stop and go traffic if you run the ac you'll get bad gas mpg. its better the roll down window in town and use the ac on freeway because with the windows down on the free way it causes drag on car in turn lowering gas mpg.