if you are looking for a backpack for messengering you might want to stick with messenger-specific bags. I've done a crapload of hiking and none of the bags i've seen were designed to fit fileboxes or big, cube and rectangular objects like the messenger back packs are.

The hiking/outdoor specialist backpacks are more cyclindrical because that works better for hiking and moving around. for small envelopes and tubes/boxes they'd work... but get an oversized run (or a series of them) and you might be hooped.

the freight/reload/chrome bags are big box shaped bags for a good reason.

FWIW: one of my friends up here has the mid-sized chrome and loves it, she lugs a fair bit of stuff around in it each day.