Yes, this has been an ongoing battle between the members, whether to use Premium or Regular fuels. I have owned 3 ES's since 1991 and I constantly kept switching octanes on all these cars, looking for increased performance and/or gas mileage, and to this day I haven't found any differences, except in my wallet.
If I found ANY differences, or if the engine didn't run as well, I would switch back to Premium.........but as of now I'm sticking to Regular fuel.