I need to know everything about the conditions prior to installing a Nest. In particular, I need to know about the thermostat and, if programmable, how it was programmed. I also need to know about living patterns. Did anything change? Was the programmed temperature higher in the summer and lower in the winter with Nest or was it set to about the same conditions?
In terms of energy efficiency. How is the house insulated? Can you add insulation to the walls or attic? Are the windows double-insulated? Do you use a whole house fan in the summer to cool the house at night? Do you have a multiple zone system so you don't have to heat or cool the entire house when you spend all day in, let's say, an office or the kitchen? Is it a two story home? Do you push hot air down in the winter in order to equalize the temperature? Do you have attic ventilators?
I see Nest as more of a neat gizmo with lots of marketing behind it than a real solution to a real problem. To be sure, the real solution requires a lot more work than bolting a piece of slick looking electronics to a wall. The payoff from a real solution would almost certainly be far greater and it doesn't have to cost a lot of money. For example, I use a $200 fan to cool my home at night (taking advantage of my local conditions). We have probably used the air conditioner a grand total of four weeks in the last three years. The savings in three years are already in the thousands and the investment was $200.