All three do something, and they do different things, which is why the reviews contradict each other — people are solving different problems and reporting on the same product.
The useful way to separate them is that a window loses heat two ways, and they are independent:
Draught is air physically moving through gaps around the frame and sash. This is the one you feel as a cold stream at ankle height, and it is usually the bigger share in an old wooden window.
Conduction and radiation is heat crossing the glass itself with no air moving at all. This is the one that makes the pane cold to the back of your hand and makes the room feel cold even when there is no draught.
Now the three options sort themselves out:
- Shrink film seals the frame and traps a still air gap against the glass, so it works on both. Trapped still air is a genuinely good insulator — that is the entire principle double glazing runs on, and the film is a cheap version of the same trick.
- A removable secondary pane is the same idea with a bigger, more stable gap and a rigid surface. Better, more expensive, more effort, and it survives being opened.
- Heavy curtains do very little for the glass and quite a lot for radiant comfort, if and only if they seal at the top. An open-topped curtain sets up a convection loop that pulls room air down the cold glass and out at the bottom, which can leave you feeling worse while the meter reads the same.
So film is not theatre. The reviews split because installation is the whole game, not the product.