I've read that 'newer' vehicles, e.g. post 2001, do a little better on the mileage, and that yes, the mileage reduction varies widely across vehicles, time-of-year [winter vs. summer], and highway vs. city.
Corn prices are sky-high right now, if they come down at all and ethanol follows suit, I'd sure like to be able to take advantage of it. To me, the very rare availability of flex-fuel in a compact car was what sparked my initial interest in the new Focus over Civic and Elantra. Of course, after driving one, it was obvious Ford nailed the "fun to drive" target with this vehicle.