At least for me, I've never claimed it to be a giant percentage. Once you've run enough E15 vs E10 and have records, its easy to see that I do get lower fuel mileage on E15. Statistically, its almost 2% lower. I'll also note that despite the higher octane rating, my truck doesn't see it that way and actually reduces the timing etc... on E15. Not just as simple as energy content.
So for me, at current prices of 3.99 a gallon for 87 E10, if E15 is more than about 8 cents a gallon less, then it makes sense to run. Many stations here are only 3 cents cheaper a gallon through...