I've got an insulated 70-oz Camelbak backpack (in the stupid-looking desert camo . . . I figure it'll reflect more light and won't get warm as quick) that I used all last summer for running and mountain biking here in the high desert of NM. Wow, it's great.
I can't imagine a leak problem if you take care of it, and extra bladders are available. (I've got several friends with them, and no one's ever had a leak.) They'll get yucky if you don't either a) dry them completely after use (which is a pain in the tush), or b) re-fill them and keep them in the fridge (they fit great along the ledge at the back of the bottom shelf). Spend the extra $3 (if you're not already getting one of the "deluxe" Camelbaks that have all the bells and whistles) and get the bite valve.
Camelbak makes an excellent product. (They have a pretty good site, too, as I recall . . . )