A while back I talked about Saving $ with a programmable thermostat
I guesstimated then that turning down the heat at night was saving me about $80-90 at the time. Thats based on using less heat during the night versus keeping the heat set at 72 for 24hrs a day.
In my mind the key with the programmable thermostat is that it automates turning down the heat and never forgets. But be clear that a programmable thermostat is not magic and doesn't do anything you couldn't do yourself by diligently turning the heat up and down manually.
Which I talk about more by answering the question : Why are programmable thermostats better?
Another point I made in that article was that contrary to what some people think : NO, turning the heat up and down does NOT hurt your furnace or cause the furnace to work harder than just keeping it on.
But there is an important detail is that if you use Programmable thermostats with heat pumps, steam heat and radiant floor heating then you need to get a special kind of programmable thermostat that is designed for use with your heating system. Typical programmable thermostats are designed for forced air heat systems and the other heat systems operate differently so the normal programmable thermostats won't work well with them.