[gfloyd2002]

I have both a tank bag and a backpack, but prefer the backpack in most cases. My tankbag is a Joe Rocket Manta, it just marks the hell out of my tank, and is too small to hold work documents, any gear, etc. I'll use it to hold keys/wallet/etc. on a short pleasure ride in the heat, but for commuting I use a MotoCentric Centrek Backpack.

With the backpack, I'm very hesitant about putting anything on my body that could be a weapon in a big crash, so I won't carry: 1) parts or tools; 2) pens/rulers/etc; 3) anything metal or plastic or glass that could break into anything sharp except my cell phone; 4) weapons, ammunition or anything explosive (/me glares at ungluck and jmcdonald).

With limitations on what you carry, I feel a close fitting motorcycle-specific backpack built with protective material (mine is 1700D ballistic), reflective logos or striping, and a minimum of straps to flap around or get caught on stuff, is a decent risk/reward and could even act as protection in case of an accident.