Product review (5 stars): This thing is AWESOME!!
This vent cover has a great design that is really effective at sealing-off the cold air that used to come into our dryer all winter.  The entire installation took about 30 minutes, including removing the old dryer vent cover.  There are some great pictures here that help you understand how it works:  http://www.amazon.com/gp/customer-media/product-gallery/B00009W3I4/ref=cm_cr_dp_cust_img_see_all_img2

Overall, this product was pretty simple to install, and made an immediate and huge improvement by blocking-out the cold winter air we've been living with for too many years!  It was well worth the $20 and I just wish I would have done it sooner!