E10 is completely safe, reduces emissions, is cheaper, and adds 2–3 octane to the blend. Florida even mandates its use. I'm not sure what then downside is...
Oh. My. God.
The emissions are only better at the vehicular level, not close if you include production. Also, as mentioned, it gets worse gas mileage. Also, it is really really awful.
Yes using it in your car has slightly lower emissions, is safe, and yes it is subsidized to the point that it is about the same cost as gasoline (also subsidized, I know), but the production of ethanol for this is a terrible misuse of resources. None of the current research seems to support ethanol as a sustainable, reusable fuel.