I use a Platypus Gravity Works water filter for the BOB.
I liked the idea of a straw type system, until we had a drought one year. Then, a number of water sources dried up. So, if I had to hump it home, those sources would not be available. Whereas with the gravity filter, I could fill up 3L (the clean bag is also my water bladder) of water and have that range (I figured 1L an hour, 3 hours range time before needing to find water again).
If I was using a straw system, I would either have to drink as much water as possible, my stomach being the storage. Or suck and spit 3L of water into a bladder. I was not into that idea.
This scenario was under the assumption of prolong drought conditions, the heat and humidity experienced during that year, carrying a pack, and hiking it home,5hrs. Lot of hills.