Insulating window films are worth it.
The insulating window films will help keep the heat in the house during the winter and block cold air from cold drafts coming in.
Installing insulating window films on your windows can help reduce your heating bills in the winter.
Also in the winter you can install some blankets over the existing curtains which helps insulate the windows as well.
Or buy some insulating curtains that you can install during the winter months too which also helps keep the heat in and your house warmer.