I use an older version of this...




...for commuting. I often extend to >20 mi per leg for workouts and ride about 4" of drop. The bag pretty much disappears from consciousness, but does prevent sweat from evaporating to some extent, even though it's well constructed to provide an offset from the back. It also obstructs easy access to jersey pockets. I don't use the hydration feature because, as a roadie, I wouldn't be caught dead.

I was a messenger in NYC for a while, back in the early 80s, and found that messenger bags don't work well unless one is sitting fairly upright.