Yes, programmable thermostats can reduce the energy used for air conditioning or heating by 5 to 30%. Programmable thermostats, while not always digital, save money by turning the air conditioner to a higher setting (or heater to a lower setting) when no one is present in the house, or in the evenings when it is cooler. You can achieve the same savings without the programmable thermostat, but you would have to remember to change your thermostat every day when you leave the house, and turn it down every night when you go to bed. In addition, if you are using the thermostat to regulate your heater, you would wake to a cold house. The programmable thermostat does all of the remembering for you once it is set. A sample of a heating schedule you might program into a thermostat is:

Wake up 6:00 am - 9:00 am 68°F
Leave 9:00 am - 5:30 pm 60°F
Evenings 5:30 pm - 11:00 pm 68°F
Sleep 11:00 pm - 6:00 am 60°F

This way your house is always comfortable and you can save money on heating. You can make a similar schedule for air conditioning.

Wake up 6:00 am - 9:00 am 75°F
Leave 9:00 am - 5:30 pm 80°F
Evenings 5:30 pm - 11:00 pm 75°F
Sleep 11:00 pm - 6:00 am 78°F or off

http://hes.lbl.gov/hes/answerdesk.html