Proponents say blending ethanol into gasoline cuts foreign-oil dependency and emissions. Opponents say ethanol production may actually increase pollution while tightening up the supply of corn that would otherwise be used for food stock. Also, some in the vehicle industry aren't convinced that ethanol doesn't damage engines, especially at higher blend levels like E15.
What can't be debated is that increased fuel economy is causing an overall decrease in gasoline use. That is great news for the environment but makes it harder and harder for the country to reach its ethanol mandates because they are largely based on flat numbers and not on a percentages of total fuel use. The current fleetwide fuel economy of more than 25 miles per gallon is about 25 percent more than it was when the Renewable Fuel Standard was first enacted. For more, take a look at the EPA's "rule summary" page on the Renewable Fuel Standard proposal here.
Earlier this year, the anti-ethanol camp received yet more ammunition after researchers at the Argonne National Laboratory estimated that ethanol production used about 12 times as much water as gasoline. So, basically, those folks could argue that more ethanol production is messing with both the country's food and water.