I ride with the biggest CamelBack they offer (sorry don't recall the name) ~170 miles commuting each week. It's big enough to carry spare tubes, tools, my shoes and in a pinch more clothes. It is not perfect, if I use the center strap I find it begins to cut off circulation to my arms on long climbs, but you get used to it. In fact I now ride with it on weekend 60+ mile rides as I can no longer stand reaching for bottles in cages.