A number of caffeinated products such as caffeinated water have used buffering agents to hide the natural bitterness of caffeine. Caffeine is very bitter. Barq’s Root Beer contains caffeine and the company says that it has “12.78mg per 6oz” and that they “add it as a flavoring agent for the sharp bitterness”
There is some question of the validity of the caffeine as a flavor agent argument for soft drinks but regardless of whether caffeine is really used as a flavor enhancer it is tough for anyone who has ever had pure caffeine to argue with the fact that it is bitter.
Caffeinated products such as caffeinated water hide the natural bitterness of caffeine. In “The World of Caffeine: The Science and Culture of the World’s Most Popular Drug” the makers of Water Joe are quoted as saying that they rely on a method developed by a flavor chemist to masque the bitterness or caffeine. (page 206)