I've gone through about a half a tank at 30% ethanol. No check engine light. It's hard to tell if there's a hit to mileage or not. My wife and her heavy foot drove the car, and I've done more local driving than normal. So far, I've averaged 22 mpg. The last tank of "normal" gas, with 10% ethanol, was about 24 mpg. But I also expect that mileage to improve, as I do only commuting this week.
Driveability is good. If there is less power, I can't feel it. I did feel a little "gutlessness" on the highway one day early on, but I haven't felt it since.
My next tank of gas will be 40% ethanol. I'm going to try and find out what the limit is, and then back off by 5% on the tank after that.