I use Camelbacks for longer rides. I was skeptical for a long time, but tried it and love it.

My long-distance bike is a recumbent so I don't have to wear the bladder; I don't even have any backpacks for them. I put mine goes in the seat-back bag, which has an insulated/waterproof pocket with drains just for the purpose.

For really long rides, I have both bladders and bottles. With the bladders you can't easily tell how much is left, and you can drink them empty when you're still way out in the middle of nowhere. If you've got a bottle or two beyond that, you know you need to go into economy mode and watch for anyplace you can get more water.
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Some competition-minded people have pointed out that with a bladder you cannot track how much you're drinking, which is a drawback for training or competition...-but for recreational riding, being able to easily take a gulp whenever your mouth gets dry is very nice.
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