I ride fairly conservatively ( for an old, retired guy), and I use ECO all the time. According to the display computer, I get 42-44 mpg. However, when I calculate mpg based on miles driven since last fill, divided by gallons required to fil, I get 30-31 mpg. My wife's Ford Focus does waaaaay better.
I really don't care that much about mpg, but I am disappointed that the display is so horribly inaccurate; it's off by 25% or so, easily.