+1 on a backpack. do your shoulder/back a favor.

I rode with a Chrome Civilian for 4 years and a custom Seagull for the past year & a half. Both bags are great, but the Seagull is truly awesome. I took it on a 2 and a half month trip through SE Asia and China, use it daily on and off the bike. It's a warrior. The only complaint I have is the same complaint I've got on all mess bags at this point: somewhere down the line, if you use one for long enough, your shoulder and back will pay the price. Especially if you use it extensively off the bike. There both great bags, and I still don't anticipate going away from the Seagull anytime soon. But a backpack purchase is definitely in the near future.