Yel, first of all camelbak started it all, I used a lot of hydration packs since 91', MULE is a compact great design yet you can store a lot(10 liters), great organization inside, easy access, your cellphone or ipod is safe, waterproofing is good too.fits comfortably, 3 liter reservoir, great for day or epic rides. Im currently using the Ergon BD1,check this:
http://www.ergon-bike.com/us,so far the best i have used, fits better, little bit bigger (12 liters), suspension is state of the art. more pricey than the Mule.I guess when it come to this, stick to the one's who knows, Oakley focuses on the Aesthetic part of their backpacks and not the application. They should improve in this department if they want to go technical, as they are behind from other manufacturers such as gregory, camelbak, golite, north face etc.