Is the Nest thermostat smart enough to turn itself on BEFORE you get home?
The Nest saves people money simply because most people can't/won't program their thermostats.  Where the Nest will mostly auto program itself based on your away/home and manual thermostat changes.

You can set the temp ranges for away as well as another emergency min/max for when the unit is turned 'off' (this can prevent frozen pipes).  The unit can also be set to start before the program schedule time to warm/cool.

But if you go out for a night, the unit then goes into a non-scheduled auto away it will wait until it detects movement before it will kick the heat/ac back on.