We bought window air conditioners for our house this summer in an effort to reduce our energy usage. Window units in bedrooms only actually. They are energy star and cost very little to operate. Our electric bill was cut by half this past month.
We still use our central air, but keep the AC set at eighty during the day and off at night. It almost never kicks on before five in the afternoon and because of our big shade trees it stays around 75 most of the day.
We can set the window units at 73 at night, which is nice and cool and still keep our electric bill down! I'm loving this.