We had ours done this year for the first time in our ownership (10y) and I doubt the people before did it. Have we noticed a difference, not really. Seeing the crap that came out was pretty terrible and ultimately it was probably worth doing once and then maybe again down the road 5 years.Had it done shortly after we bought our house because my wife has allergies and the previous owners had 2 cats. We noticed no change, but in fairness to the company that did the work, they couldn't clean everything because a couple of ducts were doubling as conduit for electrical wires.