Solar heating and perceived temperature
The conversation has gone long and isn't really that appropriate for the comments there so I've moved it here.
Summary: The approach described in the instructable is to create a thin metal panel that hangs inside of one's window, which is heated by the sun's radiant energy. Convection then carries the hot air next to the panel up and circulates it within the room. The question is whether this approach provides a warmer perceived environment than simply letting the light into the room.
As I see it, the question has two components. First, which one nets more heat energy inside the room? For an average room, the light will be scattered many times before a tiny fraction reflects back out the window. A flat foil panel as described in the instructable would have to be extremely absorptive to match this level of energy capture, because it gets only one bounce for its capture and also must capture the heat in an extremely small thermal mass. For this part of the question, not having a panel seems the likely win.
The second part of the the question is more complicated. What is the perceived temperature in the room, given differences between convection heating of the air versus radiative heating of objects and surfaces in the room? On this count I'm unsure, but it seems that radiative heat contributes more to human comfort (see this Wikipedia article: http://en.wikipedia.org/wiki/Mean_radiant_temperature). If that's true, the panels lose again.