When E10 first came out, I had a car that consistently got 40+ mpg.
After E10, my gas mileage dropped exactly by the energy ratios of E10/E0!
I never saw 40 mpg again in that car running E10. E10 has less energy per gallon and you see it in the fuel economy!
The car was a 2004 Honda Civic HX.
I never noticed a difference. But where I live, we had MTBE in the gas instead of ethanol, till MTBE was replaced with ethanol because the oil companies didn't want to get sued over groundwater contamination. And a quick Google search indicates that 10% MTBE gas has about the same BTU content as 10% ethanol gas.