Product review (4 stars): It DOES slide around and off the vent easily though (especially in a room where kids play) and ...
Works well to redirect the heat. We had to put the crib over the vent when we learned #3 is coming (we only have 2 bedrooms so the kids all share). There is only a little bit of heat floating up to the crib, but that is because our vent is wider than the extender. It is just enough to keep the baby warm though so we aren't worried about it. It DOES slide around and off the vent easily though (especially in a room where kids play) and so we used adhesive strips to attach it to the floor (hardwood laminate). I don't know how you would keep it in place on carpet unless it was tightly wedged under something. Overall we are happy with it- it solved our problem and with a little tweaking, it is a great solution!