At the heart of every hydration pack is a large reservoir or bladder. These are made from a plastic material with a drinking tube and some sort of shut off valve at the end of the drinking tube.
The quality of the water bladders varies greatly from maker to maker. Generally the knock off brands or low price ones have flimsy made water bladders that leak after moderate use or can puncture easily. However you can buy a brand new water bladder from a reputable maker and stick it in another pack, just be sure to get the correct size.
Camelbak is bar-none the leader in water bladder technology. They are tough, durable and generally come with a lifetime warranty.
In fact if you look around on the video sites you will find a Camelbak stress test of the water bladder, these things are really durable.
One of the drawbacks to the water bladder system is that they are a prime place for bacteria and mold to start growing because they collapse and won't dry out on there own unlike with water bottles.
So after each use you should unscrew the lid and empty out all the fluid from it and hang the bladder upside down to let it dry out completely. You may need to prop it open to help get it dry.
If you use a liquid other than water then be sure to rinse out the water bladder completely with water and dry. Trying to get dried on sticky gunk out of a water bladder can be a painfull process.

Hydration Pack Reviews