Alcohol contains less BTUs then gasoline. So you should always get less MPG if you run the 90/10 mix of gas/alcohol that seems to be the norm these days - as opposed to running 100% gasoline. And when we are all running on E85 it will be worse.
But there are a few benefits. Running alcohol gas from day one will keep your fuel system clean (alcohol is a very nice solvent)and you will probably not suffer from any kind of gas line freeze-up during extremely cold weather. I'm old enough to remember when you always carried a few cans of Heet in the trunk during winter.