Product review (4 stars): Solved our dryer issues.
Our dryer vent has two nineties and goes up 22 feet.  The vent cover on the roof also acts as a restriction.  Two hour drying times have been reduced to an hour (similiar to our previous house with two foot of vent to outside wall.)

Booster turns on when the dryer turns on (detects pressure in vent line.)  Booster fan is very quiet.  Cannot hear it when the dryer is running.  Only way we know it is working, is that it turns off about three seconds after the dryer.

Only drawback is that it takes up space behind the dryer.  We are fortunate to have a deep dryer closet.  Most folks should consider putting the unit in the vent inline (say in the attic.)  The unit has no maintenance.  No need for easy access.  Could be put on a shelf, mounted to the wall, above the dryer.

We have noticed a great reduction in the amount of lint buildup on the roof vent cap.  Used to build up to the point of restricting the cap.  Our guess is that the air is dryer (lint sticks less), and that there is a greater amount of air volume flowing through the line (more pressure to force lint out.)  Installed ours six months ago, haven't touched it.  Haven't had to clean the cap on the roof vent either.