Acai berries are found primarily in Central & South America. They have a nice antioxidant profile, and taste great to boot. There is apparently some research being done to explore its therapeutic potential.
But right now, I don't believe there is enough research to completely justify it being touted as a "miracle fruit from the rain forest." This is especially true of Monavie, which, like most MLM products, is probably over-hyped and over-priced. You can probably find something just as good for a lot less at your local health food store.
I drink some acai juice on a semi-regular basis, because it does seem to have some health benefits, and simply because I like it.