Like Eggman mentioned...I have been running 100% gasoline in my car for the last 10 months. I think I gained an easy 2-2.5 mpg by using it. 100% gasoline (E00) is harder to find and it's more expansive.
I just filled up a few days ago...and here's what the pump prices looked like...
You can see that 100% gasoline is $0.70 per gallon more than 87 octane E10 in my area.
Here's what it would cost to fill up with E00 vs E10 (87 octane)...
8 gallons x $3.24 = $25.92
8 gallons x $2.54 = $20.32
So it cost me an extra $5.60 to fill up with E00.
With E10 I get 50.0 mpg (400 miles per tank)
With E00 I get 52.5 mpg (420 miles per tank)
So I paid an additional $5.60 to get an extra 20 extra miles out of a tank of gas.
Let's say that 20 miles is 1/2 gallon of gas in a Mirage...or about $1.25.
That means I paid $5.60 to save $1.25.
So E00 is not an economical fuel because of its price point. But I will probably continue running it because I want to support this company for selling it and I don't want it to go away. I do run it in my lawn mower for the reasons that Mark pointed out.