I recently found a local gas station that only sells gasoline that is ethanol-free. I've been keeping track of my mileage over the last several weeks to see what the difference is between buying ethanol-free gasoline vs. most gasolines which contain up to 10% ethanol.
With a good mix of city and highway driving during my daily commute I've found that I get up to 13% more MPG using the non-ethanol gasoline. I average 30mpg on straight gasoline while gasoline with up to 10% ethanol has ranged between 26 and 28mpg.
This certainly correlates with the fact that ethanol contains a lower energy content than regular gasoline.