Today we are getting our ducts cleaned. It is time. Our house was built in 1912. We have lived here for 8 years. In the last 8 years we have done a lot of renovations, therefore I'm sure a tremendous amount of dust and debris have made it into our vents.
I've heard people comment on the efficiency of duct cleaning or the lack there-of. I have to believe it helps to some degree when it comes to air quality, reducing allergens and dust and creating better air flow. What I do know though, is if they are "sucking" a crap load of crap out of my house I am happy. Unfortunately I was at work while the ducts were being cleaning but my husband was very happy with the company. They fish a camera into your ducts so you can see the before and after and boy was there a difference!
I found this to be a good read while doing my research. I also called numerous other reputable companies in our area. I do caution you to be weary of some, which I will refer to as the "fly by night" companies. When it comes to all home maintenance and repairs do your research, if the price sounds too good to be true, it probably is.