I just so happened to be in the middle of corn country driving my wife's van, and I saw E85 for $1.67 a gallon. I took the opportunity to top it off with E85.
I got exactly 8 gallons of E85 before it clicked off, it's a 20 gallon tank, so roughly 40% ethanol.
I noticed that it lost about 4 mpg on the ride home, but no check engine light or any drive-ability issues.
Every once in a while if I'm driving this van alone, I'll add 4 gallons of E85, just to keep the fuel system clean.