I've spent a good deal of time in Argentina and Uruguay. The people drink large quantities there as well as in Paraguay. It's good stuff. I drink it at home often. You can get it at many gourmet grocery stores, import stores like Cost Plus, and even oddly many Middle Eastern markets. I agree with RockHard that it gives you a very pleasant, relaxing, yet clarifying effect. I don't know why. It just does!
Mate is traditionally drunk out of a gourd which is also called a mate.
How to Brew Mate