Bottled Water: Did the water in the bottle you just purchased really come from the beautiful spring shown on the label? How can you be sure the water in the bottle is any cleaner or safer than your tap water? How does the cost, both short term and long term, compare with other water purification options? 

Bottled water is simply water from some source that a company (or in the case of water vending machines, the consumer) has placed in a bottle for resale.  Bottled water can have minimal (or no) processing - as in natural spring or mineral water, or it can be completely filtered and demineralized to nearly pure water and then have minerals added back in to make it taste better.

But, how can you be sure the water in the bottle you 
    purchased is any cleaner or safer than your tap water?  Just 
    like with any of the other water treatment solutions,  you will 
    find reputable companies that provide a safe, quality 
    product, and you will  find companies that fill their bottles 
    using the local, municipal water.  Each company must be 
    evaluated individually (A Review of Bottled Water and a 
    NRDC Discussion of Bottled Water).  There are 2 regulatory 
    organizations, 1 trade association, and 1 independent 
    certification organization which contribute to insuring the 
    safety and quality of bottled water.

According to the University of Illinois at Urbana-Champaign, College of Agriculture, Cooperative Extension Service, bottled water is regulated at the federal level, and in some cases, at the state level. Bottled water suppliers who are members of the International Bottled Water Association (IBWA) regulate themselves and are given unannounced inspections by NSF International.  Bottled water companies can also have their product certified by NSF International.

Federal standards - Bottled water is regulated by the Food and Drug Administration (FDA), which requires manufacturers to submit samples regularly for testing. In the United States, the concentration of contaminants found in bottled water must be less than the "allowable levels" set by the FDA which are essentially the same as the maximum contaminant levels that the EPA requires municipal water supplies to meet.

State standards - Individual states must enforce the federal bottled water regulations, but they can also pass stricter standards.

International Bottled Water Association. The bottled water industry regulates itself through the International Bottled Water Association (IBWA). Bottled water FAQs. The IBWA sets manufacturing requirements, which help to ensure that bottlers meet FDA health standards. Bottled water producers who are members of IBWA are inspected annually by an independent laboratory, the National Sanitation Foundation. Through unannounced inspections, members are evaluated on compliance with the IBWA's performance requirements and FDA Quality Standards.
Not all bottled water manufacturers are members of the IBWA. Of the 475 bottling plants in the United States (in 1994), 250 are members. The label may indicate whether a bottled water comes from a member company.

You might want to check  NSF International to see if the bottled water you are interested in purchasing is certified.  The NSF Bottled Water Certification Program is an annual, voluntary certification process that includes both extensive product evaluations as well as on-site audits of bottling facilities.  The NSF testing program provides for annual unannounced plant inspections covering every aspect of a bottler's operation, from the source of the water, through the disinfection and treatment process, and including the container closure process. The company also performs extensive product testing for over 160 chemical, inorganic, radiological, and microbiological contaminants.  An interesting article from Scientific American magazine on bottled water.

The advantages of Bottled Water include:
 An emergency source of water in the event your primary
    water source fails or becomes contaminated.
 A convenient source of usually safe water for drinking
    outside of the home.
 Bottled water, since it does not contain chlorine, and may
    contain a mix of minerals to enhance flavor, may taste better
    than untreated tap water.

The bottled water industry would like the public to believe that
    bottled water is purer and safer than tap water and water
    produced by other treatment methods.  The actual quality of 
    bottled water depends on the bottling company, and most is
    probably of good quality.  High quality home water treatment
    methods (filtration, reverse osmosis, and distillation) can
    usually produce water of equal or better quality, however.

The disadvantages of Bottled Water include:
Cost - Bottled water costs anywhere from $0.25 a gallon for
    water from a vending machine to $0.50 for generic water
    jugs you purchase in a store to $1.00 - $1.50 or more for
    water delivered to your home. 
  If you purchase 10 gallons of bottled water a week, you will
    spend $260 (at $.50/gal), $520 (at $1.00/gal), $780 (at
    $1.50/gal), or more every year.
  Many of us are willing to pay extraordinary prices for the
    availability and convenience of of certain products. That 16
    ounce bottle of "designer water" you just purchased at the
    gas station for $0.69 costs $5.52 per gallon, probably more
    than twice as much per gallon as your gas! Purchasing one 
   16 oz bottle per day (at $0.69 per bottle) will cost you $248 
    per year - that's for a total of 45 gallons. If you take a minute 
    to fill your own well-cleaned bottle with filtered or distilled
    water, however,  a 16 oz bottle of water would cost perhaps
   1 - 2 cents a day, or less than $7.30 per year.
Convenience - Using bottled water requires moving and
    storing jugs or bottles of water. Water weighs about eight
    pounds per gallon, or about 40 pounds per five gallon bottle.
    Failure of a small valve can lead to a big mess, as I
    discovered after arriving home one evening after a long day
    at work to find 3 gallons of water soaking into the rug.
Environmental Impact - Producing bottles uses resources,
    and unless they are reused or recycled, they cause a waste
    disposal problem. Recycle or reuse the empty bottles, if at all
    possible.

Keep it Clean - According to the University of Illinois at
    Urbana-Champaign, College of Agriculture, Cooperative
    Extension Service,
   "If you choose bottled water after careful investigation, keep
    in mind that all of your hard work will go to waste if you
    aren't careful about keeping your bottled water clean. You
    have to be faithful in maintaining the hygiene of your
    bottled water, or you may increase your exposure to
    bacteria. Bacteria grow best in warm, moist areas. The wet,
    warm, threaded cap of an unrefrigerated bottle of water is a
    perfect place for bacteria to grow; they will begin to grow as
    soon as you break the seal. If ingested, these bacteria can
    cause gastrointestinal problems and other health risks. The
    key is to maintain the cleanliness of your bottles and store
    them properly. Follow these hints: 
   1) Store the bottle in a refrigerator at a temperature above
       freezing but less than 50 degrees Fahrenheit. 
   2) Wipe the seal with a clean cloth after each use. 
   3) Avoid any type of buildup in the bottle cap. 
   4) If your bottle is refillable, make sure it is well-cleaned and 
       rinsed before refilling. If possible, recycle the old bottle
       and obtain a fresh, sterile, sealed bottle."    
5 gallon water dispensers must also be kept meticulously
    clean to prevent bacteria from growing in the reservoir area
    and bubbling into the bottle.  Safety & Health Services
    cleaning recommendations. (pdf file)