Product review (5 stars): Works!
I didn't have to buy the expansion pack, either!  We bought a new house and a new washer/gas dryer last December.  It was taking two or three cycles to really dry the clothes, and I noticed that the clothes didn't seem very hot.  So I pulled out the manual, and it mentioned that the vents might be clogged.  I started to run the dryer, went outside and found the vent, and realized that hardly any air was coming out of it.  So we bought this.

We used the dryer as a fan and that worked well.  We got about half a shopping bag worth of lint out of the vent.  We also learned that the grid should be removed from the thingy on the outside of the house.  We hooked everything back up again and turned on the dryer.  Then the king of lint balls shot out!  Damn thing was the shape of the pipe and 3 inches thick!  Once we realized how much air was supposed to come out of that pipe, we were shocked.

7/8/14:  We did a yearly vent cleanout with this thing last month.  We tried it with the shopvac this time, and it worked great.  Hardly any lint came out, though, so we probably only have to do it every other year.