I'm another one who occasionally uses rollable thin Platypus or Vapur water bottles, but mostly only if I'm going ultra-light, carry on only, and not expecting to carry my own water most of the time. (I.e., not on a nature/hiking trip, but city jaunts.) I *only* fill these with water; not sure you could ever clean milk or juice out properly. Ick.

Most of the time, I carry a Kids (smaller 12 oz) size Nalgene bottle, but replaced the included top with a normal screw on one. No sippy cup for me! I drink a lot of water, but I find it easy to fill up most places I go in the USA. I've realized I firmly prefer the wide mouth (easy to clean), rigid sides, and non-breakable nature of Nalgene, but carrying a huge bottle annoys me too much to be worth it.

Sometimes I even use Nalgene bottles of the type sold individually for shampoo/toiletries as a mini water bottle. I have pills I must take, and a 4-8 oz bottle always fits in my everyday bag/purse, and those weigh a lot less when full. (I keep Nalgene for food separately, in my kitchen, from those I use for toiletries.)

For nature oriented trips when I'll be outdoors a lot, I have a large (38 oz) stainless steel bottle that won't break, but doesn't leave water tasting of plastic if it sits too long or in the sun. This is way too heavy for me to travel with by air, however.