I drove a 3.8L Impala for a few years and got 28-29mpg w/ regular. My GXP gets 21mpg following the same route using premium. If I'm real careful, I can get 23mpg on the highway. So, don't have high expectations on the mileage.
At 15,000/yr & assuming $2.50/gal for premium, I figure a penalty of $550/year in gas expenses.
I understand you can run regular (or mid-grade), but there should be some loss in performance and mpg.
Again assuming $2.50 vs $2.30 (premium vs regular), premium gas needs to get you about a 1.8mpg improvement in fuel economy for break even.
I've had some of the common problems, plus a wheel bearing and heat seater.
Would I buy another... probably not. Besides, in a couple years, it'll be tough to find one with lower miles.
But I'm not giving it up either. It'* a LOT of car! And a Love/Hate relationship. It beats the heck out of my wife'* CTS.