This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Backpacks are convenient, cheap, readily available, and the first
thing an Average Joe looks to for carrying a small load. They are also
not particularly well-suited to using with a bicycle. There are two
reasons I give them any positive thoughts at all. First is because
masses of less- experienced cyclists pick them up and use them simply
because they are the most convenient answer to carrying cargo. Second
is because many cyclists on rec.bicycles.misc use them and argue
persuasively in their favor. Those cyclists who do so universally note
that they use one of the backpacks designed specifically for use while
cycling or other heavy physical activity - they are designed for
lateral stability and with good back ventilation.
Most backpacks are directly next to the back, and thus have an instant
ventilation problem. A loosely fitting backpack carrying a few
textbooks can be a dangerous threat to your stability. Personally, I
think the stability problem here is a little less dangerous than
instable panniers or handlebar packs, because an inexperienced cyclist
will readily feel the instability of the backpack. Panniers and
handlebar packs can and will go instable with little or no warning to
an inexperienced or less- skilled cyclist. But instability is easy to
recognize in backpacks. There is one circumstance where load
instability will occur that may be less readily recognized by the
inexperienced: leaning in a turn. When this happens the pack, or its
contents, slides to one side of the body, creating a situation where
the pack changes the center of gravity - its weight is then pulling to
one side or the other. This is the worst possible time for this to
happen, with the cyclist already in a balancing act.
Things to look for are back ventilation, and adequate suspension for the
load (waist straps for heavier loads).
C. of G. Very Poor-Poor
Ease of Access Good
Comfort Very Poor-Good
Typical usage: Short distance/around town, Commuting
Weight carried: very light to medium (25 lbs.), more could be
carried, but would create extreme stability and control problems.