Product review (5 stars): Works as advertised.
I have a 15-foot run from the outside to a 90 degree turn, then there is about 9 feet of vertical vent up a wall. I was able to clean the 15-foot run from the outside with the dryer running with no difficulties (I also bought the 12-foot extension). I used a corded drill and rotated the brush quite slowly. I have solid 4" vents, so I can't attest to how well this works on hose-style venting. I'm not sure if the vent in this house had ever been cleaned before, but I got about a plastic shopping bag worth of lint out of the vent.

I stopped at the 90 degree turn in my vent because some moderate pushing didn't convince the brush to move forward and I didn't want to push my luck. At some point I need to clean the vertical section in the wall. To do that, I'll have to go in from the side where the dryer attaches.