In reply to WillBroad:
I've got both and they have different advantages and disadvantages.
Platypus (not a big zip one) - better for backpacking as it stands up and is generally less floppy so better for using as a water bottle in your tent porch than a camelback. Also, can replace the drinking tube with a sports cap for making this even easier. It's rigidity also makes it better for an old rucsac that doesn't have a special sleeve inside for a hydration system. The transparent drinking tube makes it easier to see if there's any furry nasty stuff that needs properly cleaning out.
Camelback - easier to fill due to large opening (can fill from streams, I guess, although I never actually have). The bigger opening also makes it easier to clean out. It's more of a pain to store 'cos it doesn't stand up in a way that will keep it more open and let air circulate around it.