Does anyone store their water bladder on the outside of their pack between their back & pack? I'm trying to find a good waterproof backpack with this but I'm having trouble.

Also anyone think of any pros/cons for this? I haven't used it on a hike but the backpack I tried to rig up with this seems comfy enough.

I'm not sure what you're describing. Most packs have a sleeve inside against the frame for the bladder. Some packs, like the Osprey Exos or Atmos, have a mesh back and space between the mesh and the frame but outside the pack bag, where a bladder can be placed.

You don't need a waterproof pack. Use a heavy duty trash bag as a pack liner and wherever you keep the bladder it won't get the stuff in the pack wet. I usually put the bladder inside the pocket intended for it, or lay it on top of everything inside the pack.