A few more words on how it works. I am sure others will chime in to tell me I kinda don't know what I'm talking about but here is how I understand it.
The A/C Compressor must be turned on in order to properly pull the heat out of the air (because that is what you are doing when turning it on, you are pulling the heat out of the air that is coming out of your vents) after this, the heat is carried to the front car where it is displaced via the condenser. Some AC systems cycle the compressor on and off in order to keep your inside air at the appropriate temp. Other systems use a small valve called an expansion valve to limit how much freon flows through the system.
The Catch is the hotter the outside air is, the harder the AC system is going to have to work. If you are bringing air into the car that is 95*, the system will have to work harder instead if you had it on Max, which will recirculate the inside air (which will most likely already be at 75*), therefore the system will not have to work as hard, WHICH ULTIMATELY will allow the motor to run easier and not use as much fuel (either by the compressor not running as much, or the expansion valve staying open wider).
With all this being said, as far as energy goes, "In Theory" you would get better gas mileage by using the "recirculate, or MAX" button on your climate control, but it does'nt always end up that way. But regardless you are going to get better fuel mileage than if you were driving with the widows open.
Mike.