Product review (5 stars): Awesome!  Saved me $30 just on the first cleaning.  Caution: requires use of brain...
This is a great product!  Did the job very easily and no problems.

The good:
- price - at $21, it's totally worth it, even if you only use it once.
- soft bristles so it's pretty much impossible to damage any part of the ductwork.
- rods are very flexible so can bend around corners
- 12' is long enough for most ducts

The bad:
- nothing

Considerations:
- if you need more than 12', you have to buy another 12'.  I wish they would sell extensions in 6' kits, or maybe an 18' brush kit option.
- This is not "industrial-grade" so you have to be a little careful when using it.  This is NOT a complaint - a heavy-duty version would cost $100 or more, and I'm happy to only pay $21 and get what I need.
- you have to turn it ONLY CLOCKWISE.  It's designed for use in a drill while spinning in the standard right-turning, clockwise direction.  If you turn it backwards, it will unscrew the rod sections and the brush will be stuck in the duct.
- use a cordless drill with a clutch and set the clutch low.  Then if you encounter resistance, you won't risk breaking the rods or brush.  It should turn very easily if you're doing it right.
- clean a 2' section of duct at a time.  Start with one section of rod hooked to the drill, run it, pull the brush out, and let all the loose lint blow out of the pipe.  Then screw on one more section of rod on and repeat until you have reached the end of the duct.  As it scrapes lint off the duct walls, it will build up in front of the brush, which is why you have to keep pulling it out and feeding it back in.  This is not a design flaw, just the nature of the beast.
- you can work this by hand, but it won't get as clean, and you will need to use more of a in/out motion while turning slowly clockwise.
- I have seen several negative reviews saying things came off stuck in the duct or it wouldn't work by hand.  I am pretty convinced that all of these people are clearly not thinking nor reading instructions before use, or have improper expectations of how such a device should perform.  An indestructible rod/brush would end up tearing the duct loose, which would be far worse than buying a new $20 brush kit.