ECO button
I drove an EX sedan a couple of days ago and switched back and forth, on-off with the ECO button. (No AC on at the time, so I don't know how that affects it.) Having the button on did two obvious things: less responsiveness in the throttle, and the transmission wanted to be in a higher gear--that is, it upshifted sooner and did not downshift as readily with the application of more power. Basically, it turned the car into a DOG, but that is a trade-off I'm sometimes willing to make in order to get better mpg.
I didn't take the car out on the interstate, so I don't know what difference ECO would make in the gas mileage when holding a constant 70 mph; my guess is, not that much. The ECO mode is probably more effective in saving fuel in around-town driving with its frequent gear shifts and changing amounts of throttle.
If you want to be compulsive (who, me, compulsive?) about your gas mileage, there is both an instant mpg gauge and the blue/green light system (blue=normal, green=gas-sipping operation) on the upper dashboard to keep you occupied.