If you're looking for the king of all hydration packs at a MUCH lower price than they are new, it's your lucky day. Adopted as part of the VIRTUS equipment system, and currently issued to the UK's military, these are the best of the best hydration packs. Here are some of the features that make these absolutely the best:
Bladders are made from Co-Extruded BPA-free PE Film, which means liquid does not leach that nasty "plastic" taste that other hydration systems have.
Bladder and drinking tubes are treated with an FDA approved and EPA registered antimicrobial agent, commonly known as "Grunge-Guard" that inhibits bacterial growth for the life of the pack.
Wash with water and hang to dry for a day. MAY BE USED WITH SUGARY BEVERAGES SUCH AS ENERGY DRINKS OR GATORADE which is not that case with most other hydration packs.
High-Flow "Storm Valve", which is also made with a 90 degree bend for ease of use and less excess tube. This no-bit design allows water to flow, and resembles drinking through a straw. Easily disassembles for cleaning, or are available for about $10-12 brand new online.
This pack allows you to wear this as a low profile backpack or may be integrated on MOLLE gear.
NOTES ON CONDITION: These are all used, so they will need to be cleaned (you should clean them anyway even if they're new). Some will have writing on them. Expect these to have wear on them, but most are in really good shape. Some of the mouthpieces are new, some are used. If you don't want to wash the used one, new mouthpieces are $10-12 bucks online and clip in super easy. If you have any other questions, feel free to ask.