E10 has caused people's poor fuel storage habbits to come back and bite them in many cases. The things you could get away with when dealing with straight gasoline, lead to failures with E10. E15 will make this issue worse. There are engines which will run poorly and and some may be damaged on E15. I'm not saying it is the end of the world, but it will cause issues with some existing engines. Also, continuing to increase ethanol in pump fuel without making a corresponding chemistry change in certification fuel while decreasing the allowable exhaust emissions will lead to increased product costs. Eventually, we will be looking at closed loop fuel injection on a walk behind mower and the corresponding price.