Hydration systems will continue to evolve as long as humans head away from the kitchen sink. One of the latest innovations for hikers is the SoftBottle from Platypus, which carries good, and not so good, aspects.
Available in 17- and 34-ounce versions, these soft plastic (a nylon-polyethylene blend) containers are extremely packable, as they roll or fold up to take up virtually no space, and are light as a feather (the 17-ouncer weighs as little as .8 oz / 22 g depending on the cap you go with, the 34-ouncer as little as .9 oz / 24 g). As a result, they take up almost no room in your pack, and don't add to the overall weight (until filled, of course).
There are two capping systems -- one, the familiar screw-on closure cap, the other what's termed a HyperFlow Cap with an integrated bite-valve. Which you go with depends on where you want to expend your effort -- unscrewing and holding a cap while you drink, or twisting the HyperFlow Cap a quarter-turn or so and biting on the valve to drink. If you're meticulous about your pack's weight, the regular screw-on cap weighs three-tenths of an ounce less than the HyperFlow Cap.
Pros: Lightweight, highly packable, no Bisphenol A (aka BPA), an organic compound that some believe messes with your hormones when leached from plastics and ingested.
Cons: Drying out the bottle after you return from the wilds can be time-consuming, and cleaning can be a bit more involved than simply resorting to a bottle brush (ie: Use hot, soapy water and rinse thoroughly with hot water. For tough cleaning jobs, add 1/4 cup of baking soda to 3/4 cup of water per liter volume of your bottle or reservoir and shake for 30 seconds. Add 1/4 cup lemon juice, shake 10 seconds, and vent by loosening cap away from face. Repeat shaking and venting three times. Expel as much air as possible, cap bottle, and allow to soak 20 minutes. Rinse three times with hot water.) Also, depending on which water filtration system you use, these small-mouthed containers could be a bit tricky to fill out in the field.
Pricing: $7.95-$12.95 depending on size and cap system.
Bottom-line: A good day-hiking option, but for longer treks a larger volume hydration pack, coupled with larger containers (ie. Platypus Water Tank, a DromLite Water Carrier from MSR) for in-camp use might be a better choice.