I like the flask, too, but I've lost a couple. After you get used to it, you can judge the amount you're supposed to take in pretty well -- for me one big slurp is about right. You could also dilute it that way if you didn't like the texture or wanted it a little more liquidy.

I do really like using my Camelbak because then I can drink as I please, instead of having to wait for an aid station for water, and then gulp it down all at once, which makes my tummy feel sloshy. It might not cause side stitches, but it feels weird. I can take in a whole gel and a few long sips of water, but beyond that, it feels icky.

I generally go by the "more than an hour of running = carry water and food" rule, but I live in a not so humid climate. I do also have to make sure that I'm hydrated ahead of time no matter what or I get to the point where my temp starts rising and feel thirsty.