Caffeine...in water.  You can barely taste it!
I work nights, so having a consistent flow of caffeine is important.  I can only drink so much coffee, so this was the next best thing.  It has the slightest after taste that you really don't notice if you don't drink other water before or after.  Unfortunately, I keep pretty hydrated at night, so I taste the difference.  Knowing that the flavor is an intentional additive makes it ok.  Overall, I like them but if you're picky about your water, you may want to skip it.
Rating: 4 stars