I have always wanted one, but I believe that the best way to make money is through the sale of other items.

Have a donut/coffee shop, and if you can stick in a subway, do that there. Sell the gas as low as possible and make a profit of 5cents for every gallon, pretty much be the cheapest station around, then rake in your profit from the other things you sell.

There is a gas station out here that consistently sells their gas 10cents cheaper and while they have around 20 cars pumping gas, the other stations around will have 1 or 2 at best. They might not make profit from their gas, but from selling cigarettes, candy, cookies, and the fried chicken, they more than make up for it.