I've wondered the same thing. I bought a brand new Camelbak last year and it leaked right away. My inside of my bag was all wet.....not good. Going with a MSR or a Platypus. Guess I better check out the Source as well.
I use platypus.
A Camelbak bladder weighs roughly 1.5x what a similar Platypus bladder weighs. Maybe 3-4 oz more weight. The platypus hoser is a few bucks cheaper than the cheap Camelbak is on sale. If you really think you need a big lid for cleaning access or whatever the Big Zip is the same price as the Camelbak stuff and just a touch lighter.
Why buy something that is heavier, likely more expensive, and more likely to fail?