I have a Camelbak bladder and have had no problems with it. As has already been mentioned, blow back into the tube in sub zero conditions to prevent ice from blocking the tube. Insulated sleeves are available. I know someone who puts a disposable hand warmer in the bag next to the bladder in winter to help keep the water above freezing too.
I keep mine in the freezer when not in use as it prevents bacteria and mould from growing inside it if there's any moisture left in it. Cleaning mould out of the tube is a nightmare even with the special long brush so this avoids the problem. If you do want to dry it out the plastic hollow practice golf balls work well and are cheaper than a specialist bladder drying frame.
Magnets and compasses are something to be aware of, I wouldn't want to store a compass near it although I presume the magnet is on the shoulder strap and not in the compartment of the bag, and be aware of not holding the compass near the magnet when using it. My Deuter day pack has a velcro strap for the water tube and this works just fine.
On the magnet subject I once heard a story of someone who packed their gear into a car boot so that their compass was next to the magnets of the speakers in the parcel shelf.