I get 4-5 mpg better mileage but I definitely feel a difference in power. Not a lot but it is there.
Just curious where you believe you feel the power difference.
It automatically drops out of the ECO mapping if you apply more than 30% throttle. That's why I try to remember to engage it every time I ride. That way it's in ECO when it might matter, and doesn't do anything when I feel the urge for power.
Honestly, I can't think of a reason NOT to have it on. I just wish it would remember the setting when you turn the bike off.