It depends. Some smoothies are quite high in calories and sugar, with up to 600 calories and 27 teaspoons of sugar, depending on the cup size.
But made right, smoothies are a nutritious and a delicious way to get some of your daily recommendation for fruit. They're usually high in vitamin C and often full of calcium, vitamin A, fiber and protein, depending on the ingredients.
It's easier to control calories and sugar, and boost nutrition when you make smoothies at home. Use ingredients such as fresh or frozen fruit, low-fat or fat-free yogurt, milk or soymilk, maybe a banana, and a splash of 100% fruit juice.
If you visit a smoothie stand, order the smallest size and choose one with few ingredients. Consider skipping "add-ins" since many increase cost with unproven health benefits.