Use denture cleaning tablets. If they're safe for granny's dentures, they're odourless and safe for your camelbak.
Plus they are a hell of a lot cheaper than the ones Camelback sells.
It is worth investing in the cleaning kit they sell though. It comes with 2 brushes, one for the resevoir and one for the tube, and a hanger that goes inside to dry it out.
A cheap and easy option is a three drops of thin household bleach (unscented) to a litre of water. Swill it around for a about a minute, flush through the hose. Rinse well then store in your freezer to save having to dry it.
Its only ever going to have water in it. Puritabbed, or tap, There shouldnt be any bugs in it anyway. It might taste minging but then its only ever going to taste of plastic or chlorine anyway. Heavy rinsing and strong alternative flavour, Vanilla essence or cordial left on the mouthpiece seems to do the job for me.
Chlorine it. If you don't like the taste, then soak it out with a baking soda/water mix overnight.
Yes, you need something in there that can kill bugs. You might not have many big bad microorganisms flowing out of your tap, but they do live in your mouth and they will grow in your mouthpeice and tube and resivour. Trust me.
Well, you can see how much water is left in the bladder. The bladders also have markings on them indicating how much water is left in them. If you are wearing one and don't want to take it off, you can squeeze it or bounce it around a little. After you've worn it a little, you can tell by the weight or how much the water swishes around how much you have left.