According to a study released by the Environmental Working Group, bottled water is no better than filtered tap water, despite being considerably more expensive (not
to mention the carbon footprint of bottles and transportation!).

• Many bottled waters are municipal water sources that have been
  filtered or otherwise purified.  As much as I complain about Cameron,
  TX tap water, I'm not sure I'd rather be drinking water from Las Vegas
  or Houston.
• Bottled water manufacturers are not required to disclose test
  results for contaminants, unlike municipalwater.  The report showed
  that faith in bottled water purity is not always well-founded
• The same chemical profile is found in many bottled waters as in tap
  water.  Those images of mountain springs don't have much to do with the
  source or the purity.

I geeked the study, and the it passed the transparency test of
having the actual data available.  Although somewhat alarmist in its
executive summary, the basic truths have not been twisted.

The bottom line:  You are better off financially and environmentally
buying a good water purification system for your own tap water.