I first ran across yerba mate in Chile and Argentina where proper consumption requires a dried gourd and a nickel plated filtering straw called a bombilla.
Mate, in parts of Argentina and all of Uruguay and Paraguay, is more popular than coffee is in the US.
Unlike Red Bull and Rock Star, the caffeine delivery in mate is gradual on the way up and the way down. Guayaki, the high-end mate provider, has a great little video that explains some of this stuff and pitches their corporate responsibility. Guayaki is expensive as mate goes, but their various offerings of drinks, sodas, loose mate and tea bags are all exceptional. I buy the cheap stuff.
Mate has been linked to some esophageal cancers. I am not too worried about it. I believe that some or all of this is caused by the temperature and quantity of the mate as traditionally consumed in Uruguay, Paraguay and Argentina. You can purchase loose mate in most natural food stores and in latino markets. Loose mate is very inexpensive on a per drink basis.
Here are some links that talk more about mate: