So I constantly go between liking panniers and liking a backpack. Panniers are rad cause there is no weight on your back or ass. They kind of suck because jumping curbs or going over roots on a trail is kind of difficult.
Well you need to look at the Vaude Siena 40. IT is a nice bag for carrying all your supplies, has a built in rain cover and external shoe pockets, a helmet holder and a few pockets. It also has a webbing ring on the bottom for securing your blinky LED.
But most important about the Vaude bag is the arc suspension in the bag that keeps your back from getting nasty and also keeps the contents of the bag from diggin in to your back. Its literally a small frame that keeps the bag off your back.
Its really nice using this bag to commute. I has 3 outside pockets for keeping your wallet and keys. And the big main pocket, it holds as much a medium chrome or timbuk2 but in a much more comfortable manner.