A wild cherry slush may seem like a harmless treat on a hot day, but its sugar content can drive the calorie content up. A 20-ounce slush contains between 300 and 358 calories, depending on from where you purchase it. (ref1 and 4 - note a sonic slush large has 24 ounces/McDonalds 22oz, so I did the calculations) All of these calories come from sugar, making it a carbohydrate-heavy drink that provides no nutritional value.
A 20-ounce cherry slush contains between 73 and 95 grams of sugar. The American Heart Association suggests you consume no more than 24 grams of added sugar daily if you're a woman or 36 grams per day if your a man. The sugar in a slush is equal to 18 to 24 teaspoons of sugar. (ref3)