The windows are large in our house, so they're more important than they might be in a house with smaller windows. However, the rooms without double glazing are those upstairs which we don't really need to keep particularly warm.
For some time we've been weighing up the pros and cons of spending money on glazing for rooms which don't necessarily need to be kept at a particularly high temperature, and eventually made the decision to go ahead with the work.
We don't expect it to make the same big difference to our heating bill as cavity wall insulation did, as the thermostat in the house is downstairs and relatively unaffected by the temperature upstairs. However, it will presumably mean the entire house is better insulated so have some effect on the ground floor as well, and of course it also means the end of condensation on the bedroom windows, which froze on the insides in the winter and made it difficult to open the windows.
Yesterday it happened. New glass in the first floor windows.
So far, our heating bills have been lower each successive winter since we've lived here, as each time the house is a little better insulated than the previous winter. It's a pattern I'd like to continue.