This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Handlebar packs or bags are a subset of panniers, but I treat them
separately because they have many avid proponents, and have enough
individual considerations that they need to be treated
separately. And, really, there are two types of handlebar bags or
packs. There are bags, which strap to the handlbars without the
benefit of a frame, and packs, which use an external rigid mounting
frame or rack of some sort. I'm not going to distinguish between them
for this article, and I will use the names interchangeably. Handlebar
bags have two distinct advantages: they can be used to carry a map
that is always visible, and they are highly accessible. They also have
distinct and potentially dangerous disadvantages. They are extremely
easy to overload. When they are overloaded they readily cause
instability and a steering effect on the handlebars that can be
dangerous. Their mounting systems tend to be less than ideally stable.
I have used them for their advantages, and I find that to be a small
advantage, indeed - too small for me to bother with. But, they have
folks who love them, and who really appreciate the advantages I
mentioned. So if you like the idea, I will say this: don't overload
them. They are suitable for a jacket or two, a camera, a cell phone,
and a map, and nothing more. They are not suitable for school books,
laptops, or other dense items. They have enough space to pack this
way, an inexperienced cyclist probably wouldn't even think about it,
they would just toss in a couple of textbooks because there's enough
room for them. A couple of textbooks can easily weigh 10 pounds, and
this would be an overload!
As for me, I'll pass on looking at my map all the time. A fanny pack
or pockets will be fine. The one exception would again be long
distance self-contained touring. Long hours in the saddle would mean
my comfort level demands as little constraint on my body as
possible. So, then, combined with whatever else I used for the real
load, there would be a place on my bike for a handlebar bag.
C. of G. Very Poor
Ease of Access Very Good
Comfort Very Good
Typical usage: Short distance/around town, commuting, day
trip/century, touring (self-contained)
Weight capacity: Up to 5-7 lbs. would be typical.