I don't like filling up so often but I'm not that worked up about mileage. I bought a fully loaded 3 year old STS at the end of 2008 for 1/3 of MSRP. When I was car-shopping most low mileage 3-year-old performance luxury cars (like my STS) were priced a lot more than $22,000 I paid (including CPO).
Figure a worst-case-scenerio at $5 per gallon, 15 MPG & 10,000 miles a year. In three years, I would spend $10,000 on gas. On the other hand, lets say I bought a performance luxury car getting 25 MPG. I would spend only $6,000 on gas but I would have spent a lot more for the car; probably $30,000. My net loss would be $4,000. ($22k + $10k = $32k vs. $36k = $30k + $6k)
If you figure the total cost of ownership, the STS is still a good deal. What's more, hybrids and electric cars aren't the good value that some would have you believe. (Even after the wasteful government subsidies.) Their break-even point is far past 100,000 miles even at much higher gas prices.
Be happy! Gas is less than $4 per gallon and an STS gets a lot better than 15MPG.