I try to avoid having it off. When I'm stopped at a light and the engine doesn't shut off, the first thing I check is the ECO button. Turning on ECO allows the engine to shut off. The reduced power air conditioning as well as not allowing the use of the electric heating elements in ECO mode should increase economy much like a two-stage furnace/air conditioner in a home.

At least for us, we are comfortable with ECO on, so the amount of savings doesn't matter -- we are thankful for any we get.