i use a columbia backpack... nothing special about it either. of course, i'm carrying books(accounting) in it so it's alittle heavy.
side note: i just bought a rear rack so i'm going to start putting the backpack on it.
if you're set on a backpack.. i think anything with chest strap and a waist strap will work fine. then again, these two features aren't really needed.
what distance is your commute?
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Bikes: C'dale, Trek, Cervelo and Santana