Sarah - Acai berries have antioxidant properties but you can also get really good sources of antioxidants for a lot cheaper in the form of things such as blueberries and green tea. I've heard that you have to eat the fresh berry (or frozen) to get the properties and that juices aren't as good and pills definitely are just a waste of money.
They won't help you lose weight or anything like that but they are just something that if you can, you might want to include them as a part of your diet. For most people, they are too expensive and other sources of antioxidants are just as good.
You can't out-exercise poor eating habits.