I go to Polis Sucos every day. Actually, I go twice a day: once in the morning before I walk to work and then on my way home at night after the gym.
The guys who work there know my regular order: açaí com banana y proteína, sem sryop. In English, that's açaí with banana and protein powder, hold the syrup. (This is the extent of my Portuguese.)
They also sell juices (sucos), sandwiches of all kinds, and tapioca, which are like savory crepes filled with egg, cheese, and/or meat. I've yet to try one these.
I go so often because of the location; it's only a block from my apartment. But it's also the best fast food around. Order from anywhere along the counter and in under three minutes you've paid and are eating your food.
Polis Sucos has become part of my life in Rio and I'm going to miss it when I leave.