Product review (3 stars): Well, it works but . . .
It directs the air like it should, however, I wish it were wider (it's a couple of inches too short for the width of my vents) and there should be a way to lock in the length once you slide it out.  Our vent is under a sofa and, if we happen to hit it with our feet, it retracts a bit.  I put duct tape on it (good ole duct tape!) but shouldn't have to.  It's overpriced for what it is (flimsy piece of plastic) but there weren't any other options out there that I could find.