The ducts at my house were cleaned this fall and I noticed a big difference in the air quality and the dust residue on the furniture. Before there was heavy dust all the time, I would clean and the next day it looked like I hadn't done anything. After the ducts were cleaned, there is still dust but it is not heavy like before and I can go several weeks between dustings. The cleaning even made a difference in the air coming from our vents, it is blowing stronger. I guess before it was being slowed down by all the crap in the duct work.
I would definitely recommend getting your ducts cleaned every couple of years. We had waited too long and there was such a build up of dust in the vents that it not only looked very gross, it was not a healthy environment for my husband who suffers from asthma.
Carleton Clean Air Technologies did a superb job cleaning our duct work. I have no hesitation in recommending them to others and they are definitely going to be the people I call when the time comes to redo the cleaning of our ducts.