Product review (5 stars): Great product
Now here is a product that worked better than I expected.  What they don't mention is that the kit comes with an adaptor that allows you to go through your dryer ductwork either from the outside or from the inside while attached to the shop vac.  It turns out I have about 18 feet of ductwork (it doesn't look that big) but the kit allows you to extend the brush 12 feet.  With the adaptor I was able to clean most of the way from the outside and then run the brush from the inside out to clean the rest.  The brush did a better job with my smooth ductwork than my corrugated expandable kind.  But even the corrugated ductwork is much cleaner than it was before I started.  My ductwork has three 90 degree bends that the kit was able to negotiate without problem.