Not sure if I'm allowed to post a link, but a Canadian consumer news program did a story on premim VS regular. To no ones real surprise, if you car doesn't need it (per the manufacturer), you are wasting your money. In fact, one very artsy lady driving a Honda Civic, who thought the environmental benefits outweighed the price was stunned to find the pollution form the exhaust is worse with premium! They had an expert put a brand new GM car on a dyno and measured all that could be measured. Essentially, the computers in our cars adjust to the fuel. In most cases there was no benefit and in some there were worse scenarios.
If posting the link is verboten; my apologies.