very common with the radonneurring (LONG distance riders) crowd and of course tourers as well. roadies probably laugh at them, since they don't ride long enough to really need one.

on long rides i use either an ortlieb handlebar bag (the "classic"), or a carradice "junior" saddlebag. but a handlebar bag is nice because you can get to it easier, even while riding.

and here's one for you: wind-tunnel tests (done by Bicycle Quarterly) showed that handlebar bags can increase your aero-ness, because it acts as a small fairing (wind shield). gotta find that link..