I drive a 1996 Honda Accord EX. It gets better mileage on Regular than Premium, but better mileage on Premium than E10 Regular, even after factoring in the price difference.
Ethanol separates from gasoline in about a month. If the gas sat at the gas station for a few weeks before you bought it, it can separate before you use it up.
Ethanol will eventually damage the rubber and other components of your non-ethanol-safe fuel system. These components can be replaced with ethanol safe ones.
Small engines in lawn mowers, snow blowers, chainsaws, etc. seem to be more susceptible to damage from gasoline with ethanol in it.
In some places, the high octane gasoline does not have ethanol in it but some cars do not run as well on high octane gas because the gasoline does not ignite at the most efficient time for the engine. Where I live, high octane gasoline has no ethanol, and mid grade gasoline has half as much ethanol as regular grade does. Apparently our mid-grade is a 50/50 mix of regular and high-grade. I don't know if that makes sense, but that is what I've been told by 4-5 different people.
My car does not get as good mileage out of E10 gasoline. I experimented with using high octane gasoline and found that, even though high octane costs more, the extra miles per gallon I got was worth the extra cost. This will likely not be the case in cars that do not run well on high octane gasoline.
In my area, we do have a few gas stations that sell ethanol-free gasoline.