It depends if its Sierra mist, no HFCS.
I'm not a be healthy nut, but the body doesn't respond well to processed food especially if its consumed in high quantities. Processed foods are broken down to its simplest forms, so it gets absorbed a lot faster so the body doesn't get a gradual increase in blood sugar. This throws off the body chemistry, cause the body treats 200 calories of fruits a lot differently than 200 calories of Coca-Cola. It becomes a vicious cycle, because some of these foods do not produce any type of satiety, so you are hungry again. Also, the stomach treats all these chemicals in processed food and drinks as irritants, so it makes you feel hungry when in fact you just need some rolaids. Also, you can't eat 1,500 calories worth of vegetables without feeling like you are about to explode, but you can eat 3 slices of pizza (1,200 calories) with no problem.
Sierra Mist is not good for you at all. That's just advertising nonsense. The difference between "natural flavors" and artificial flavors is nothing.