It means that you will get fewer miles per gallon, in my case, it is an 8% loss in my LT1 (350cid) Caprice, the price might drop a few cents a gallon as it is <currently> pretty heavily subsidized by the Governement. Oh...yeah, almost forgot, it takes more oil energy to produce it than it 'saves' by replacing 10% of your gasoline. You remember, that gasoline that will cost you mileage...
It is great for ethenol producers, not so great for everyone else, except for the people we buy oil from as it takes more energy to produce than what burning it releases.
Ethenol is a very fine octane booster, not so good as a fuel.
No engineer in the World would say this is a good idea, it is special interest groups that pushed money at politicians and pretty graphs that showed how 'wonderful' this garbage is.