Gaining

Even something like fruit juice, which has added sugars. They don’t fill you up too fast and so you can easily drink as much as you please and get all needed calories.

Only if you want to raise your risk of metabolic syndrome (diabetes and the like.) It’s why you see a lot of gainers who want to gain quickly going the heavy cream route, as dairy is good to create subcutaneous fat (I’m not sure I spelled that right) which is healthier and jigglier; a feature sought by many gainers. See the heavy cream threads for more info which speaks to the difference. Sugary drinks are fine in moderation, but ill advised to use as your main source of calories.