Mythbusters (the TV show) proved it's MUCH more than .25 mpg, it's more like 25% worse fuel mileage.

I track my mileage on all my cars by resetting the odometer each time I fuel up and calculating the mileage for each tank. When I run the air conditioning I rarely see a loss of mileage of more than .5 mpg (on vehicles that average at least 20 mpg). No way have I ever seen even a ten percent decrease in mileage.