If, like us, you are on well water, chances are that your water is as clean, if not cleaner than bottled water. If you have a contaminated well, that is a whole 'nother issue. I recommend having your water tested periodically.
If you are on public water, it is the law that they regularly test their water and make the results of those tests available to the public. You can look up the results of your local water utility here.
Bottled water that is labeled "purified water" is taken from lakes, rivers, or underground springs and treated, all of which makes it almost identical to tap water.
Other brands of bottled water is just tap water in disguise. In a quote from Jim Shepherd, Dasani's group director of research and development, he says that "Coca-Cola tested Dasani in hundreds of focus groups until it hit on a markedly crisp quality, achieved by adding magnesium sulfate, potassium chloride and sodium chloride to purified municipal water." Which is a fancy way of saying that they simply add salt to tap water in order to make it taste better.
Not only is it more expensive per gallon than gasoline (if that's not a kick in the gut, I don't know what is!), bottled water incurs a huge carbon footprint from its transportation. Meaning that it takes energy, gas, etc. to bottle and transport the bottles. 1.5 million barrels of oil, in the US alone, are used to make water bottles. What's worse (or maybe not, they are both pretty bad) 86% of these bottles are landfilled or incinerated.
And finally, if all of this has not convinced you, let me leave you with one plea. Please do not re-use your plastic water bottles! It is a noble gesture, I know, it will save you money, it will save the bottles from piling up in the landfills. However, this is a dangerous thing to do! The most common plastic used to make water bottles is #1 polyethylene terephthalate (PET or PETE). If reused, they can leach chemicals such as DEHA, a known carcinogen, and benzyl butyl phthalate (BBP), a potential hormone disrupter. According to the January 2006 Journal of Environmental Monitoring, some PET bottled-water containers were found to leach antimony, an elemental metal that is an eye, skin, and lung irritant at high doses. Also, because the plastic is porous you'll likely get a swig of harmful bacteria with each gulp if you reuse #1 plastic bottles.
I would recommend investing in one of the re-usable water bottles on the market. My family uses Kleen Kanteen and we love them. Others report success with the Sigg Bottles.