Product review (5 stars): this thing worked great as described
First of all, this thing worked great as described, and my dryer is again running at full exhaust volume.  That said, thought I'd add a note of caution that may help somebody from repeating my mistake.  As is noted in the questions sections above, do not, I repeat, do not run it in reverse.  I started cleaning my 20-or-so feet of duct, and about halfway through felt a bit of resistance.  Figured I had hit a clump of lint, and--OK, so maybe I had a beer in my hand at the time--thought it would be a good idea to back it off a few inches and re-approach the minor obstruction.  What I did, as other posters have noted is a risk, was to unthread the brush from about 12 feet of extension sections.  Tried pushing it through; no luck.  Tried my homeowner-grade plumber's snake; too short.  Ended up having to buy a second one of these kits, and, voil!, the second brush pushed my mistake brush out to the duct vent.  Great product; works exactly as described; my stupidity was the only limiting factor.  Our clothes dryer is running like the day we bought it.  Bottom line--buy it and keep it turning clockwise!!