I generally try to avoid using a bag, but sometimes it's necessary. So I use the free one I was given, as the original owner was cleaning out things he never used. This one was still in a plastic bag and never used, not even once, until I used it:

http://www.hotref.com/Imago-Compu-Backpack-p-3447.html

I like it and at first I was skeptical of taking another's clutter, fearing it would suffer a similar fate.