Yes, using the a/c or c/c does use power and more petrol or deisol.
If your car is idling and you switch on the a/c or c/c, you will hear the engine revs drop and the sound of the engine change as the a/c or c/c unit takes power from the engine. If your car has a smallish engine and you're driving uphill with the a/c or c/c on, try turning it off and the car will go faster!
Whatever the cost - its great on a hot day when you're stuck in a jam or going fast on a motorway. However, when just cruising, I personally prefer to have the windows or sunroof open. (You pays your money and takes your choice).