I've been using a Camelbak milspec 3L bladder for about the past 5 years and never noticed an odd taste. Maybe that's just me.
I washed it out good with soap & water before the first use and only put water in it.
No drink mixes. These can be food for bacteria, especially if they have any sugar / carbs.
Also, when done with it I clean it with soap & water really good (bladder, hose, & mouth piece) and hang to dry completely before putting it up. Sometimes I will also use the Camelbak cleaning tabs in addition to the soap. The hose can take days to get completely dry. If you put it up wet, you are asking for trouble.