i don't want to start an argument but i am a believer. i have done it before and the results have been good. i can't say it was good every time but mostly it was. some of the contractors however, can be con's. i know alot of members here think it's snake oil and we have had a few long threads about it. not all ductwork needs cleaning, and the contractor should be honest and tell the customer if it is not needed. what is it that makes you think it's a con? i don't know what kind of machine it was we used, but it had an air hose that would reach all the registers and ductwork, and a big vacume that was on wheels. i was impressed by how well it worked, but maybe i just didn't know the differance.
I agree not all air ducts need to be cleaned. I personally inspect the air ducts with a video system with the potential customer and try to educate them about their system. Unfortunately those $79 scammer companies with their bate and switch tactics stain the industry.