I always worry that places like Jamba Juice will have too much sugar in their stuff. I make my own at home with some ice, fruit, juice, and sometimes sherbert. The portion sizes are smaller, and it's less expensive to buy canned stuff in light syrup and frozen fruits to add. You get more for your money.
In terms of the hierarchy, I know oranges are good on the IR diet, as they have a low glycemic index, so perhaps orange-based smoothies are a place to start. I believe watermelon is on the not good side of the spectrum.
Here's a site I found quickly about the GI and fruit. It's a chart so you can compare:
http://www.diabetesnet.com/diabetes_...emic_index.php