Think that bottled water you're gulping is better for you than the free stuff out of the tap? You're not alone in your misconception.
Might it taste better to you? Sure. It may have fewer minerals in it, more minerals in it, or different minerals in it than your tap water, which affects the taste. Do you prefer to drink water that's had the fluoride removed? Fair enough -- though your children's teeth might not thank you for it later [source: Kids Health]. Are you of the opinion that mineral water has health benefits that regular water does not? More power to ya.
Safety, however, is a different matter.
Municipal water sources are highly regulated for safety, with mandatory contaminant checks in happening hundreds of times per month; bottled sources are evaluated more like four times per month. And those evaluations, along with safety standards and levels (and definitions) of purity, are regulated mostly (and in some states entirely) by the bottled-water industry itself, meaning it's often voluntary.
Next, making a slippery issue worse ...