bought this for the spartan ultra beast in sept because I needed something that could carry a decent amount of water but also have room for an MRE, minimal first aid stuff and gels but was still small enough that it wouldnt get too much in the way while doing the obstacles. It is comfy, I can run, hike, climb, and fit a ton of stuff in it. It sits up off my back and I get nice air flow, the rain cover is great (took it out in the rain less then 24 hours into ownership) and it can fit myself (5'3"ish and 120lbsish OR my husband 5'11", 175lbish). The water stays cool and it is easy to drink out of, no leaking or mess.
I purchased this bag to replace a Camelbak that I wasn't thrilled with.I easily fit 3 liters of water, my rain jacket, pump, spare tube, multi tool, tire levers, headlight, back light, energy bars, keys, cell phone, id, and sunglasses. And the best part: it never feels heavy! The magnetic bite valve attachment is a huge plus as you don't have to search for the tube while riding. I also love the harness / frame system for airflow.Only complaint: On Osprey's Zealot bags, they added zipped pockets to the waist belt and harness shoulder straps. My friend uses this bag, and while I really like my bag better, these small features make be a bit envious. Otherwise, a fabulous little riding pack!