One warm day I went into my local outdoors store with my camera bag and proceeded to stuff various water bottles into the side pocket. I'm sure the staff thought I was absconding with their gear, but what I was looking for was the perfect fit. The pocket was such a slender size, that not much would fit into it and I needed something to keep me hydrated during hikes.
Plastic water bottles are too wasteful and too noisy, giving off a crinkling noise if you moved to get a photo. Aluminum bottles were too bulky and would not fit. I wasn't having much luck until I slid a narrow Platypus Softbottle into the pocket.
I purchased a 0.5 litre Softbottle (which also comes in a 1.0 litre version as well) which is a lightweight 22 g and is about 12 cm wide by 29 cm tall.
The seam along the sides and bottom give it enough structure to stand on its own when full, and prevent it from flopping over under its own weight. The Softbottle is well designed, with a slightly curved middle, allowing you to single-handily grasp it while having a drink without it slipping away. The seams also help the Softbottle stand up to lots of abuse. I could accidentally step on it and have no fear that it would spring a leak. The cap (which you can purchase replacements for if you lose one) has very tiny ridges on it to assist in opening or closing it. A very subtle design that was a nice touch.
A year later I no longer have the camera bag, but the Softbottle has become a staple in my outdoor gear collection. Because of its convenient size, I use it for numerous things from bringing water for the pooch to the dog park, to always having it handy in my daypack by the front door. Even in the winter, it is slim enough to tuck into an inside pocket so that the water doesn't freeze and still provide hydration while snowshoeing. Best of all, when you've consumed all the water inside, the Softbottle can just roll up into a tiny tube.
The Platypus Softbottle is compatible with their entire line of accessories such as the HyperFlow Cap or their Push-Pull cap adding extra functionality depending on how you intend to use it. The bottles and accessories are BPA Free, and never had a plastic taste to them. The only downside I could find with the Softbottle is that it is difficult to fill with a water filter.