It has just a hint of flavor, is not at all sweet, and is equally as hydrating as water. In addition, I find that there is a major difference between fresh coconut water and those that are commercially packaged, so try to get the real stuff if you can. Once you've had the real coconut water, the ones that come in juice boxes -- even if they're considerably more expensive -- have an unusual "plasticky" taste. Might be because the water is affected by the packaging overtime, and they use preservatives and whatnot to maintain its freshness.
Fresh coconuts are not that hard to find, and are usually plentiful in Asian supermarkets, but they may be a little tricky to extract. I know some people actually drill holes into the coconut (and by some people, I mean me), but if anyone knows a simpler way, I'd greatly appreciate the advice! It's well worth the trouble.