For many years, the idea of a durable, plastic Nalgene water bottle was the best thing going for hydration. You could kick it, freeze it, abuse it and keep rockin'. But when you needed a drink you had to stop, pull it out, unscrew the cap, and then probably drink a bit more than you should, and reverse the process to put it away. Effective, yes, efficient, no.
A hydration system is simply a way to drink smaller quantities more frequently, made up of a reservoir, drinking tube, and valves. You can grab the mouthpiece, open the valve, drink a mouthful, and close the valve, all without breaking stride. The plumbing can be added to an existing pack, and most backpacks are built with a slab pocket for the reservoir and ports for the drinking tube. Buying a pack designed for the system, however, means the reservoir will be protected from your cargo and can be accessed more easily. Hydration packs will also have more activity specific designs, well suited for cycling, running, or paddling.
Many outdoor brands have hydration products, and some, like CamelBak and Platypus, specialize in systems and bags. There are a wide variety of designs to choose from, but some basic elements cross all models:
Durability - Look for a tag identifying the use of Cordura nylon, widely recognized as the most durable fabric available. At the least you want the bottom of the bag to survive being dragged across a parking lot or two.
Water resistance - The body fabric should be shiny on the inside. This indicates a polyurethane coating that will keep water from penetrating the whole cloth. Unless all the seams are sealed, however, the bag is not waterproof. Remember that ice or refrigerated water in your reservoir will sweat and soak through the fabric and seams just as if it sprung a leak. very cold water is not a very good idea for high-exertion activities anyway, so most experienced hydration system owners make do with room-temperature water.
Longevity - Compare the size and weight of the straps and hardware. Wide, well-padded shoulder straps, big zippers, and reinforced stitching indicate a bag built to last. Lighter day packs are frequently outfitted with less-substantial findings, and though the fabric might hold up, if the straps hurt your shoulders or the zipper brings out your 'adult' language, it's just not worth it.
To make sure the reservoir and plumbing last, be sure to use only water in your system. There are several sugar-free electrolyte tablet products on the market, but they must still be rinsed out and will leave flavor residue in the plastic. Between uses, either clean, rinse, and dry your system thoroughly, or fill it completely and store it in the fridge, where microbial growth is much less likely.
Capacity - It's easy to put just 2 liters of water in a 3 liter reservoir, but will you? How much will it weigh when full? Do you really need that much water? Some people prefer to have more capacity, but each liter weighs a kilo, so a full 3 liter reservoir weighs 3 kilos, or 6.6 pounds.
Cargo - If this bag will be used primarily for your cycling workout, what do you really need to carry? For day hikers, room for a shell and lunch will be important, and a runner will do better with a waist pack. Only take what you need, and you'll get more done.
Features - Hydration packs are designed with specific users and activities in mind. It's all up to you, but as long as you are clear about defining your needs, you'll end up with the right bag for the job. Some things to keep in mind:
Cyclist packs are very low profile and have pockets designed to hold a pump, energy shots/bars, and may even have a bungee web sized to hold you helmet.
Runner's packs are usually waist-style to keep the back open and keep the water weight on the hips. Tend to have very small cargo capacity and high visibility colors.
Paddler's packs will integrate with your PFD or strap down to the deck of a kayak easily. These are usually just sleeves to protect the reservoir and allow it to be attached easily.
Hiker's packs are day packs on the smaller-capacity end of the spectrum and are light-duty compared to non-hydration specific products, which frequently can be upgraded to hydration very easily.
Whatever your decision, keep in mind that the value of quality will remain long after the satisfaction of savings fades.