Most of the time these days I use panniers, but on occasion I still use my Chrome Metropolis. I give the Chrome Metropolis top marks; massive capacity, completely waterproof, and about as comfortable as I imagine a courier bag can be. The shape of the strap is more ergonomic that Timbuk2's and has decent padding.

I have friends who ride motorcycles and some of them use this bag. In fact, one used to be a motorcycle messenger and she used a Chrome Metropolis. The Chrome is completely waterproof, you'd have to drop it into a pool to get water inside it. I've used it for years in Seattle and never had any water inside it. Mine's nearly a decade old now.

You'd be amazed at how much capacity it has, and the added benefit of its waterproofing is that keeps hot items warm and cold items cool for quite some time. Chrome's unique strap makes it easy to get on and off once you get the hang of using it, which takes no time at all.

We have a couple of Timbuk2's kicking around the house, including a nice backpack one, but if I hop on a bike that lacks pannier racks or am looking for a bag to take on the bus or on a trip with me, it is the Chrome that gets the nod.

Reading your original post, it sounds like maybe the Metropolis might be a little large for you, but IIRC Chrome has a few other sizes. IIRC, someone mentioned in a thread recently that the quality of the Chrome bags has gone down recently, you might want to search for thread.