1) Bottled water is not as stringently regulated. Your tap water is tested for E. coli and other pathogens and must show quality reports. 22% of bottled water has more contaminants than your tap!
2) It takes three times more water to make bottled water than what's in the bottle to manufacture the bottle itself. Since I live in an area with water shortages, this worries me. Since water is most likely to be the limited resource of the next hundred years, it should worry you, too. The more you drink bottled, the less there is to go around.
3) The oil which makes up the bottles for the water could power 1 million cars for a year. Only 1 in 5 is recycled. The rest? On the side of your freeway, in your trash, plugging up your landfill, and in the ocean and waterways.
4) The other side of "going green": Bottled water costs $10 a gallon on average. Tap water costs less than a tenth of a penny a gallon. Why spend so much green on a less green alternative?
The main excuse I hear as to why bottled water is good is that it tastes better. Okay, I'll give you that one. The water that comes out of my tap tastes slightly of mud. But there's an easier, greener fix to that than buying wasteful bottled water. It's called an activated charcoal filter, like the Brita filter. You buy a jug and a series of filters and create your own purified water on demand. The best part about Brita filters? They've partnered with Gimme5 to create completely recyclable filters. If you can recycle all plastic in your curbside bin like I can, that's an even better option.
The thing that held me up in my transition from bottles was actually more simple. I don't like the Kleen Canteens' opening. I had to look long and hard for a canteen that I liked enough to use. If that's your problem, investigate why you like certain bottles and not others. For me, I needed a sports top that let me not have to pause and let the air come back in. I found this for free(!) at a school giveaway, and now it comes everywhere in my purse. Outdoor stores and the internet are useful here, too.
So give tap water a chance. Both your wallet and your planet will thank you.