I'm also a fan of 2 liter pop botles. Sturdy, don't leak, easy to stow on top or in a pannier. Also lets you distribute the load a bit better then having 1 gallon bladders. A funny thing I've observed is that pop on sale is often cheaper than bottled water. I've been known to drink the odd bottle of coke, so this works out in my favor. Even if you buy the pop, pour it out and then refill the bottles from a tap, you still end up money ahead.
Cheers