Relatives in Canada have them - along with internal window quilts. It gets so cold that the glass can crack without them.
If you can manage to close them at night (and perhaps in unused rooms) they make a huge difference in heat retention. So, inward-opening windows, ground floor (if you don't mind going out) or automatic only, I'd assume.
Some neighbours here have had insulated roller shutters installed on their first floor bedroom windows. With the expected rise in heating costs, I'm considering triple glazing in our coldest room - the one above the garage.