Product review (4 stars): Works well, Easy Install
This seems to work pretty well. Took me about 15 minutes to install (and that included finding my step-ladder). The switch is a bit flimsy, but it's not horrid. It's not going to heat your whole house, or even your basement, but every little bit helps and it's better than pumping the hot air into the backyard.

Note, that there is a gravity flap on plastic flap which I assume is there in case you let the lint filter get clogged up. However, it doesn't take much pressure for that flap to be pushed up which means that about 25% of the heated air is going to escape outside no matter what position the switch is in. I would not suggest taping that flap down though...just don't be shocked on a cold morning if you see a little heated air coming from your outside dryer vent.