I'm no Chemist or expert of any kind really, but I've been using ethanol "enhanced" gas since the 1970s (not all the time, mind ya) in various vehicles and my observations are:
For real old cars (before the Gas Crunch of the mid-'70s), ethanol could attack the gaskets in the carburetor, the fuel pump diaphragm, &c. Not all cars, but it wasn't all that rare. Seems that the older the car, the worse things got.
For cars between then and the introduction of ECUs (ECM in Z-talk) the engines seemed to "tolerate" fairly large amounts of ethanol. The better the electronics they had, the better job they could do, but not quite there.
For most modern cars, 10% is no problem. I've had to put 10% in my '09 and haven't noticed any ill effects.
HOWEVER
Ethanol has fewer BTU per gallon - ~114,000 for regular unleaded vs ~82,000 for E85 (couldn't find a value for E90, but you get the picture). You have to burn more fuel to get anywhere. I haven't noticed much of a difference in power with my butt dyno, but haven't really made any comparisons - I seldom use full power and when I do, I've always gotten enough to make me grin.
Ethanol and water love each other. Especially in higher concentrations, it can pick up water (and some of the crud it has with it) and carry it to the pump, filter, injectors, &c. For a newer car that hasn't had time to collect condensation, this is seldom a problem, since most gas has a little ethanol in it anyway and the water gets carried out in tiny amounts.
All that said, I try to get 93 octane sans the ethanol, but I don't get obsessive about it. If I get anything below 93, I try to keep my toenails out of the radiator until the next fillup. I had to buy 90 octane once - just got enough to baby it home and then filled it up with the good stuff. I try to avoid anything above 10%, just on general principles, but finding 0-10% hasn't been a problem so far.
Keep in mind that all of that is just my personal observations. Nothing scientific about it (except for the BTU/gal figures and I rounded them).
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