Personally I use premium in any car I drive.
The cars just run better on premium.
Personally, I get the following from using premium...
Better gas mileage, lower emmissions, easier starting, more power, and a cleaner running car.
For me the price difference is negligable, but then again, I don't have to pay for my gas, so I will use the best there is.
My car was designed for premium, so I use it. plain and simple.
I tried 89 once, and the car ran horribly.