I just got a great Jansport bag at Target. It was only about $30 and has all kinds of separate compartments, an interior hook for my keys, an elasticized "secret" compartment in the back (PERFECT for a changing pad), water bottle "nets" on the sides, several small zipppered pouches in the front, etc.
It beats any diaper bag PERIOD. And, it's SO comfy and well balanced when I wear it. That's the biggest disappointment I've had with "diaper backpacks". They are NEVER comfortable to wear.