Have any independent studies been done showing how well, or even whether, duct cleaning works?
CMHC, the Canadian quasigovernmental organization that insures high ratio mortgages, has a lot of online resources about home ownership. Their page on duct cleaning is very negative and cites specific, controlled studies by the EPC and CMHC themselves that found no appreciable difference in air quality before and after duct cleaning, and notes that household dust generally has nothing at all to do with the HVAC system.
They do, in an effort to be fair, note that there can be special circumstances where duct cleaning would be worthwhile; if the ducts are suspected of having mold (esp. after getting wet) or if you have reason to believe there’s junk or debris in there, such as might be left by lazy construction workers.
Isn’t most household dust dry skin flakes? Maybe if you have a really dusty house, you should invest in skin lotion instead of duct cleaning. 
We built our house and for most of those years, had a yellow lab.
About 5 years ago we had the ducts finally cleaned out.
Being a nosy body, checked out the truck thingie where our debris was now at and it was like OMFG LOOK AT THE DOG HAIR!!! and dry wall dust. BUT THE DOG HAIR!!!111!!