I have used ECO mode on and off. If you are doing city driving, I recommend not using it as you may be constantly stop and go. As ECO mode adjusts the throttle, it make the car a tad sluggish going from 0 mph to 25ish mph. I recommend to use it when you are traveling at a faster speed such as 45 and up and it is consistent, such as highway driving. I use it only during highway travel as the speed is consistent. You will notice alot of saved MPG's during highway travel with ECO mode on.
Source: From my experience and personal opinion.