Just to put one more out there…I have a Camelbak Blowfish. It holds 3L (100oz) of water and has an expandable storage compartment. It has 720 cu. in. or 990 cu. in. depending on whether it is expanded. It doesn’t look quite as cool as the M.U.L.E. because it doesn’t have as many little compartments on the outside, but it is cheaper and holds more stuff.
I like mine so far.