Their coupon says they'll clean your ducts for $49.95 which includes 10 vents, 1 return and 1 main. That implies 10 supply or return air vents, 1 return air duct and 1 main air duct. WRONG! They will clean only the 10 supply air vents, 1 return air VENT, and the main supply air DUCT. They DO NOT clean any return air ducts. The dirtiest ducts - and the ones that likely need cleaned - are the the return air vents and the return air duct(s). Those are the ones that are full of dust and debris.
What they do is cut a large hole in the supply air duct above your A/C coil to which they attach a large HEPA vacuum. (They patch the hole later) This supposedly will suck the dirt out of your supply ducts. To augment the suction and clean the supply vents, they then take an air compressor and dislodge the crud out of your supply air vents so that the vacuum sucks the dirt away. They use no tools - no brushes to actually clean the inside of the ducts. You would be correct in assuming that the ducts are not really very clean once they are done, since no cleaning tools were used to truly clean the ducts. I got dirt and drywall dust on my hand when I reached into the "cleaned" duct. Admittedly the duct was cleaner than before they started.
Sounds simple, right? 1st problem: The vacuum does not know it is supposed to suck air only from the supply air duct, as there is no isolation between that duct and the return air duct attached to your furnace - your filter and blower motor are seated between the two ducts - so the vacuum is always sucking from ALL open vents in your home. The vacuum motor is relatively small compared to your furnace blower motor, so the vacuum doesn't have much suction, and since they don't use cleaning brushes, the ducts don't get very clean.
2nd problem: The vacuum sucks from not only the supply air duct but also the return duct, right through your furnace filter. When they use their compressor to clean any return air vent, all that vent debris heads right into your furnace filter, plugging it FOR SURE before their vacuum ever sees the dirt! I have a high efficiency filter that runs $30 at Lowe's to replace. It was totally plugged when they were done.
3rd problem: They will want $85 to $199 to "do the job right" for basic cleaning when they arrive. They have a big list of optional services. Instead I paid them their $50, grabbed my shop vac and cleaned out all the remaining return air vents after they left. These people are con artists - beware!