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Try this: drive 60mph on normal then switch to Eco. Do you notice the rpm's dropping? I don't. Eco mode isn't overdrive, it just slows the throttle response which is why people describe it as "sluggish". This only saves gas during acceleration.
Where eco mode can help during highway cruising is that it can prevent accidental acceleration from slight movements of your foot. This is due to its lack of sensitivity and can save gas.
I use eco unless I'm feeling frisky and have empty or nigh-empty roads to enjoy. I used normal rather than eco during one tank and noticed quite a difference. I tend to have a heavy foot, so lowering the throttle response seems an effective way to improve fuel economy. It's really not too bad anyway. You can still get a nice amount of acceleration if you drop your foot enough.