Product review (3 stars): It works well, but only after slight modification.
I was having trouble with humid air getting into my dryer; if I forgot about a load of clothes overnight, it would be damp and musty in the morning, and obviously need to be washed again. Even without laundry in the dryer, the door was covered in condensation in the morning.  This was caused by the flapper on the outside of the house no longer sealing properly and thus needed to be replaced.  Since it was going to need to be replaced, I figured I would go with a type that better seals out the outside air.

As others have noted, the manufacturing quality of this seems lacking.  In order for the float to actually seal the opening, I had to sand/file off all the rough edges. Although, I'm glad I didn't have to remove material from the housing itself, since the housing itself just feel flimsy, although at this point I have nothing to suggest that it will not be durable.

However, after these modifications, it does a good job of keeping the outside air out of the dryer vent.  There is no longer any condensation and I have no problems with clothes that smell of mildew when left in the dryer.  If I had to do it again, I'd probably order the similarly designed, more expensive Heartland vent, as it seems that it does not carry the same manufacturing-related issues.