Product review (2 stars): The rod broke inside the duct - hours to get it out.
I have a dryer duct that's about 35 ft long with three turns, so I ordered one with the 12' extension kit (GARDUS INC #R-3203612 12' Dry Vent Extention Kit), figuring I could clean part of it from one end, and part from the other.  I used a 3/8" drill to turn the brush (you need a drill - forget about turning it by hand), keeping the drill on the low speed setting, and slowly fed the brush in, adding the three foot rod sections as I went.

I got about 12 ft into the duct OK, but when I added the next extension, the brush head just sheared off.  I tried fishing it out with a wire snake, but gave up after a while, and spent the next three hours disassembling the duct to get the brush out - lucky for me the duct was mostly exposed - if it had been behind drywall or in the ceiling, it would have been a whole other story.

In disassembling the duct, I saw that the brush had done a pretty good job of cleaning the straight sections, but there was still significant buildup in the bends that I had to scrape out by hand.  The white plastic rod had sheared off about 4 inches from the bristles but was not obviously stuck on anything.

I guess if you have a small section of duct, it will probably work, but throw in a couple of curves, and I think it's really pushing the limit at 12 ft.