I have personal experience with E85 in both NA and FI cars. My experience has been with daily driving, drag racing, and dyno testing. I will tell you right now IN GENERAL you will lose 20% gas mileage. However E85 cost approximately 20% less than premium.
Depending on your car and the power you desire to make, this is actually not as expensive as most people make it out to be. If you plan on running a bigger turbo or most boost your going to need upgraded fuel pump and injectors. The only additional cost to run E85 is the difference to a larger cc injectors, normally less than $100. With that being said a good tuner, someone who has experience with tuning E85 will tell you they can easily make an additional 6-8% more power, often times even more.
In my opinion there is no reason not to run E85. We have had E85 here for about three years and I will never go back to premium. Ethanol is better for the environment, and allows us to produce more power. Cost is basically the same, although you do have to get out of your car and fill up 20% more. Anyone who has experience with E85 will tell you its all worth it.
BTW we run E98 in our FDSM making 1005RWHP, who needs VP Import for $18 per gallon when we can buy E98 for $4.50....