Yes air con definately use extra fuel as on my dads scenic 1.8 16v with climate control it makes quite a difference if using it, although the climate system automatically uses the air con to demist the windows although it does do it in a minute or less
Also when abroad we hired a daewoo matiz 800cc with air con and we used to call it the turbo button as when your going up a bank with air con on it would start struggling so press the button to turn it off and all of a sudden it would have enough power, it is a great feature to have though id love it in my next car