Product review (5 stars): Much better than the three or four shutter design
This is just what I was looking for. Much better than the three or four shutter design.  First of all I have not installed it yet but I have looked it over well. When I saw the ad with the top "flapper" open I assumed that that was where the hot dryer air came out.  Not So!! The exhaust air actually pushed open a cylinder inside the housing and exits out the bottom of the vent. This took away my only concern that the top cap could get frozen shut.  The top cap dose open to facilitate cleaning out any lint that might get caught in there. The cylinder that I mentioned looks like it will do a good job of blocking the outside air as well as any critters that might be looking for a new home! The vent seems well built of rigid plastic and is sturdy.
Again, this is just what I was looking for. I will install it on the first decent day and will update this review only if there is a problem.