Product review (5 stars): updated to 5 stars after 1 yr operation
Update 2/12/14.  After 1 yr+ operation, I have to give the Heartland 21000 dryer vent closure a 5 star rating.  It performs without all the problems that the flapper-type vent closures typically have--getting stuck open by lint, or by rusting of the springs.  When the dryer is not operating, the Heartland closes tight & keeps out fumes from lawn movers, etc.  In 1+ yr of operation, it has never stuck open & never gotten clogged by lint.  The only problem encountered:  during an extreme cold & ice storm, the Heartland "valve" froze in place--apparently due to condensation buildup.  But it was easily loosened.

As to my original downgrading of the Heartland due to install problems:  I agree that install would have been a quick easy job IF the correct dryer vent pipe had been installed in our house.  However,  previous house owner had cemented-in the vent pipe coming to outside & it had no crimped end extending outside.  That's why we had such difficulty.  So if you're lucky enough to have a vent pipe outside end with crimping, it will be a "5 star" install for you.  If not, you'll have to adapt it like we had to.

Success!  Finally got it installed and functioning but it took two of us 1.5 hrs.  Why?  (1) the product description and instructions say that it will fit on a 4" diameter vent pipe, but it won't.  The pipe receptacle on the Heartland vent contraption is just barely 4" in diameter; so a 4" diameter aluminum vent pipe will NOT slide inside the Heartland receptacle for the pipe.  To get a 4" diameter vent pipe to fit into the receptacle on the Heartland dryer vent, we cut 1" slits in 3 places on the aluminum vent pipe opening.  These slits allowed us to squeeze the end of the vent pipe into a slightly narrower diameter than 4", so it could be pressure fitted into the Heartland pipe receptacle.  (2) My house has lap siding, and the dryer vent exhaust hole in the side of the house wall was located such that the square base of the Heartland contraption partially overlapped the house brick and partially overlapped the siding.  So we had to adapt a piece of styrofoam to fit under the Heartland base so it would sit level.  (3) the instructions say that 6 screws are included with the kit, but there were NONE in the package anywhere.  So we had to use our own screws.
Now that it's installed, it's working fine presently.  And I like its design better than the more common metal "flap" that opens when dryer exhaust air flows through the exhaust duct.  Over the yrs, I've had several different brands of that style, and after a while, the flap on all of them would not close tight when the dryer was off.  Because they wouldn't close tight when the dryer was off, they let lawn mower fumes, etc from outside get into the house.  This Heartland vent closure contraption looks like it will prevent that problem.  It's made out of thick plastic that looks like it may holdup well over time--we'll see.