Interesting. Giuliano Cuete and Carlo Olcese have the exact same post about pretenders and being NADCA certified, word for word. So how do you become NADCA certified? And does that mean you are not a scam artist or a "blow and do" ripoff? hmmm, interesting. The EPA says that under normal conditions, a properly maintained system should NEVER need it's ducts cleaned.
Because an air conditioner moves heat between the indoor coil and the outdoor coil, both must be kept clean. This means that, in addition to replacing the air filter at the evaporator coil, it is also necessary to regularly clean the condenser coil. Failure to keep the condenser clean will eventually result in harm to the compressor, because the condenser coil is responsible for discharging both the indoor heat (as picked up by the evaporator) and the heat generated by the electric motor driving the compressor.
I took off stars for two reasons. Minus one star for issues trying to use the $50 for $25 voucher I purchased through Yelp. Although there was no minimum stipulated, the office wouldn't take it for the initial $59 diagnostic fee. If I used the 10% off First service that they offer, then they won't let me use the voucher on the actual repair bill.  I was stuck with paying the full $59 diagnostic fee.
Important Tip: Don’t just pay attention to the high-end of the price range! – which is what I know you are all looking at, but also look at the low range. If a contractor is cheaper than this, ask “why?” Usually it is because they don’t have liability insurance, workman’s compensation insurance, are unlicensed, all of the above, or worst yet, they don’t use HVAC technicians. You are spending a lot of money.  Make sure that they are licensed (In California, check here at CSLB License Checker) and insured.
An air conditioning system, or a standalone air conditioner, provides cooling and humidity control for all or part of a building. Air conditioned buildings often have sealed windows, because open windows would work against the system intended to maintain constant indoor air conditions. Outside, fresh air is generally drawn into the system by a vent into the indoor heat exchanger section, creating positive air pressure. The percentage of return air made up of fresh air can usually be manipulated by adjusting the opening of this vent. Typical fresh air intake is about 10%.
Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.