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.

My PAC came today. Finally, it was stuck in customs for 5 days.

Have yet to ride with it, will be heading to work soon. Amazing piece of work. Won't make any judgments for a week or two until I break it in a bit.

Which one did you get? I got an ultimate. Very good size. You would think it would be a huge bag, but it really isn't. But fits a lot.

Mine came in today- - -a "Pro Light Large" . . .it's hugely awesome, with emphasis on huge. I could a slide case of beer and a dog in it and still have room for my lunch. It's really far too large for my commuting needs. I'm going to see if I can exchange it for something a bit smaller. That said, I really like the design and construction.

I just bought a medium Banjo Bros commtuer backpack. It has plenty of room for a pair of shoes, pants, shirt, socks, and undershirt (I have a locker for shower supplies but It has room leftover after I put that sutff in it).

It is not a messenger bag but an actual packpack. So far I like it (day 2), but I still need to test it in a downpour to see if it is actually waterproof as it claims. My only gripe is that it currently sits a little high, slightly obstructing my reward vision. However, I haven't fully adjusted it to where I want it yet. The cost was $80 shipped...you should be able to find many other reviews if you google it.