A four-year study by the NRDC (whoever they are) concludes that bottled water is mostly good stuff, but rarely better than tap water. They claim that the water from your faucet is produced under more stringent rules than the stuff that comes in bottles. For example, whereas city water must be tested 100 times per month, bottled water plants need test only once a week.
Why do we think bottled water is better? Marketing executives at Pepsi and Coke are trying to sell you a healthy alternative to soda, so of course they'll advertise it as great-tasting and good for you. They're not trying to compete against tap water, though they don't mind if you somehow get that impression. But don't be fooled.
There are good reasons to drink bottled water, such as the convenience while traveling. But at home or at a restaurant, it makes much more sense to get regular, plain ole tap water.
[for more details, see this CNN story]