When we moved into our first house, we had the ducts cleaned. The house was built in 1969 and was not a big house. We bought it in 1990 and I don't think the ducts (it had floor registers) had ever been cleaned before. The company we had do it had this thing about collecting the debris into a container so you could see everything that came out. (show it to you empty first, then after when they are done). I was amazed at the pile of sand, dirt, grit, dust, a few small lego pieces, lint, cat and dog hair, bit's of paper, a penny, etc. All in all the pile was almost 5 inches deep. Our airflow improved immediately. I can't say the house had a smell to it before, but I think it smelled really clean and fresh after the duct cleaning.
I can't remember what we paid for it, but I thought it was worth every penny!