To add to what’s been said:
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While I’ve never designed an automotive engine, I’ve designed enough other things to respect the automotive designers when they say “use premium fuel only”. They are saying it deliberately. They are the ones who ran the engine through many hour of simulation, followed by actual testing and measurement. They are the ones who know the ranges of the anti-knock sensors and the air/fuel/timing maps. And they know exactly what happens when the octane is too low (under all operating conditions). I would be foolish to assume I know better than they do just because I read a few web sites or because my buddy did and his engine is still running, or whatever reason I had.
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The difference between regular and premium seems to be about 6% less (eg: 25 cents less @ $4.25/gal). If your vehicle gets 25 miles/gallon, then any drop in mileage greater than 1.5 miles/gallon means you’ll be wasting money by using regular. It’s not hard to loose 6% in mileage when dropping to regular for a vehicle requiring premium.