Just this month, I was running E85 for a couple tanks. I got 19.8 mpg on E85, and my normal is 21 mpg on 87 octane. While this is the winter blend of E70, it is still at least 70% ethanol. That is a loss of less than 10% in mileage. So if E85 is 10% or less expensive than 87 octane, it is worth it. If you compare that to premium, then the difference is even more aggressive. Especially considering E85 has an octane of 105. Around here, E85 is 2.89 while regular gas is 3.49. That's right around the 10% break even point. If you enjoy an aggressive tune which requires premium, then E85 is quickly more economical.
I didn't loose as much mileage as I expect actually. I've read studies that show E85 can improve the burn characteristics such that you get a more complete burn. That will result in improved gas mileage. If you actually tune for E85 by increasing the compression and timing advance, then you could recover nearly all of the mileage loss.