Firstly it's a very popular misconception that air conditioning is a summer only thing, and that it's all about cool. This is wrong. Air conditioning is about dry and de-humidified, and is just as beneficial in winter as it is in summer. You can use the A/C with the heater on hot just as effectively as you can on cold. The mugginess in the air will be drawn out leaving the cabin nice and fresh.

Air conditioning works by compressing, condensing and evaporating a refrigerant gas R134a. The gas boils at a very low temperature, -26 degrees Celsius. Under normal operation the compressor compresses the gas against an expansion valve (later in the system). The compressed gas becomes hot and at high pressure which then enters a condenser. Air passes through the condenser which allows some of the heat to be dissipated into the air which in turn causes the gas to condense into a high pressure liquid. The liquid the leaves the condenser and passes through a receiver dryer to allow and moisture which could damage components to be safely collected. The moisture free liquid then passes through a tiny nozzle (expansion valve), on the other side of which is a much bigger space. The liquid then becomes low pressure and subsequently the temperature falls considerably. The low pressure cold liquid passes into the evaporator which is inside the car. Heat from inside the car is drawn to the cold evaporator which then causes the low pressure liquid to evaporate into a low pressure gas, which in turn is drawn back to the compressor where the whole process starts over again.

If you don't run the system for long periods of time the gas will never turn to a liquid. The seals in the system can then dry out and deteriorate, causing the gas to be lost.

Air conditioning in my opinion is the best feature on a car and I run mine all the time. Modern cars use variable displacement compressors which are very efficient. The difference it makes to fuel consumption is barely noticeable, if at all. However, not using the A/C will almost certainly mean you'll drive with the windows open. This will increase drag, which definitely will have an effect on MPG.

All A/C systems will lose refrigerant over time, and so it's worth having the system re-gassed every so often. During this process the system will be vacuum tested for leaks, and will give the opportunity for a leak detection dye to be added, as well as a small amount of extra compressor oil to replace any that may be lost. Very often you'll be surprised when the refrigerant is evacuated quite how much has been lost, and when the system is recharged with the correct amount how much better the system performs.

Don't wish to tempt providence, but the aircon on my 2009 i30 (used often all year) still blows ice-cold on coldest setting. Not been re-gassed yet.

If you use the de-mist or dei-ce function the Aircon runs to dehumidify the car. Regards Peter