While most commercially-bottled water is considered safe for human consumption, "Studies have shown that chemicals called phthalates, which are known to disrupt testosterone and other hormones, can leach into bottled water over time. One study found that water that had been stored for 10 weeks in plastic and in glass bottles contained phthalates, suggesting that the chemicals could be coming from the plastic cap or liner" (Bottled water, 2011, NRDC). Other contaminants have been found in some popular brands of bottled water, and no study has found that any brand of bottled water is substantially safer than tap water that satisfies basic, federal environmental standards. Even if bottled water is safe, the obvious question arises: why pay for something that you can get for free? Bottled water is expensive, and surely consumer's dollars could be channeled into more worthy areas of the economy, such as buying higher-quality food (or saved in the bank)!...

Carrying a reusable container for water generates far less waste, conveys the same hydrating health benefits to the consumer, and does not pose any additional health hazards to the drinker -- at a far lower price.
References

Bottled water. Natural Resources Defense Council (NRDC). Retrieved January 12, 2011 at http://www.nrdc.org/water/drinking/qbw.asp#plastic

Plastic recycling facts. (2011). Earth 911. Retrieved January 12, 2011 at http://earth911.com/recycling/plastic/plastic-bottle-recycling-facts/