i've used many a bag in my commuting and each has it's ups and downs. my favorite are chrome bags, mine is a model called the Backbone. it's huge and heavy. reliable as hell though, a chrome bag will last you for life and is waterproof in even the most rugged conditions. but back sweat is an issue and chrome has yet to find a way to remedy this. i myself don't mind as i just change when i get to work. what i would recommend would be an ortlieb backpack called the 'velocity'. it vents better than any bag i've ever tried, it's waterproof, light, and fits so snug to your back you won't know you're wearing it. but for a cheap remedy while still holding a decent amount and battling the elements check out the timbuk2 'slacker' backpack.