The only research done that I have found on duct cleaning, outside of the duct cleaning companies themselves, has been by the Canada Mortgage and Housing Corporation -- EFFICIENCY OF RESIDENTIAL DUCT CLEANING -- CMHC Project No. : 93-2-001 January 1994.
Basically they found that when the ducts were exceptionally dirty, like after a renovation, it did some good. In general, simply cleaning what you can reach with your own vacuum cleaner at the outlets, the cold air return and around the fan motor will do almost as much good as most duct cleaning. Of the duct cleaners, the ones that did the most good were those who actually opened the ducts to get access inside, and beyond each obstruction, as well as using a back flow skipper which bounced around dislodging the dust while blowing it back towards the point of entry and drawing it into a high pressure vacuum cleaner usually located in a truck outdoors. Chemicals used to kill fungus were highly doubtful.
Generally their study did not support the strong claims of the duct cleaning industry.
CMHC information sheet CE29
About Your House: Should you get your heating ducts cleaned?
Then more recently, in 2012, the Institute for Research in Construction of the National Research Council of Canada produced a protocal for testing of duct cleaning to try and bring some order into an un-regulated industry -- so if you want all the details:
"This protocol is a test method to evaluate the effectiveness of duct cleaning (DC) and impact of DC activities on indoor air quality (IAQ) of commercial office buildings. The intent of this protocol is to provide standardized IAQ monitoring and sampling methods that can be used both pre and post DC activities to ensure that building occupants and operators have consistent and reliable data to make the necessary decisions regarding post cleaning building occupancy. Although the protocol’s primary objective is to standardize the IAQ evaluation methods, it also includes testing surface cleanliness of HVAC systems."
You can download this protocal here. This is all for commercial buildings, which usually have rigid metal ducting. Most residential Heating and Air Conditioning systems also use metal ducting, although almost all Air Changers and many other residential systems use flexible ducting. I have found no good information on the real ability to clean the very irregular interior surfaces of flex ducting. I will look forward to finding any good third party research on the effectiveness of the cleaning flex ducting. The generalized continual use of biocides remains as questionable as was brought out in the first research.
I am skeptical of this entire industry and would love to be proved wrong.