Well for starters, if you only put water in it, all you have to do is take the pack out when you get home, drain whatever is left, extend the arms that keep the pack open (kind of like sunglasses) -- and then just hang it upside down.

I assure you I'm a germaphobe and there's no grossness to my camelbak. In the case that you are putting orange juice and stuff in there, you can just flush it with water. They also sell cleaning kits, or recommend using bleach then flushing it thoroughly and allowing it to dry. Essentially I wouldn't buy one used, but if you bought it new, there's no reason the hydration pack can't look the way it looked when brand new once it's a few years old.

That's just me though. I always try to be a little uncool just because I don't want to look like I'm trying too hard.