Product review (5 stars): how many sweaters worth of lint can you get out of your dryer duct?
you will freak out.  I had my son stand outside under the vent to see how much came out while I took a leaf blower to the opening in our laundry room.  There's about 20 ft of 4' duct in our house.  We've lived there for 20 years.  He was completely covered with lint, so I figured I should order this cleaner thing.  It's easy to use and has made a huge difference in how long it takes to dry and how much heat comes out of the laundry room.