Putting 4 gallons of E85 in right when the gas light comes on results in about 35% ethanol, and almost 93 octane. It's about a $7.50 per tank savings, and doesn't really lost much in the way of mileage. This is the ethanol sweet spot.
If E30 were more widely available at blender pumps, it would be very compelling for cars that require premium fuel. It would be a nice savings.