I get my water bottles free from supermarkets. They come with Coke, Sprite, Fanta, etc, in them.
I don't like the hard bottles (Nalgene and the like) because when they are half full they slosh, and I hate walking with "slosh slosh slosh slosh" coming from behind me. Soft-drink bottles can be crushed when half full. They can also be crushed under the compression straps on the side of my weekend pack (it doesn't have mesh pockets on the side).
When a soft-drink bottle gets a bit grimy and the little black spots start growing on the bottom, I throw it in a recycling bin and buy another bottle of Coke.
However, the narrow neck makes them a pain to fill from a waterfall. The wide-mouth bottles (either Platypus bladders or Nalgene jars) fill a lot quicker.
B