Other options here:
Reload - An old messenger staple.
Freight Baggage (have one of these and its really awesome) - handmade in SF, designed for messengers by messengers.
Seagull bags - Never seen one so can't comment but his other bags are pretty awesome.
My lady has a Project Tortoise one and loves it to pieces. An added bonus is that Yasu, who makes them, is a really really nice bloke, with great attention to detail and they've been tested with london messengers.
Obviously the only downside to a backpack is that you have to take it off to access the contents, which can be a pain sometimes esp when you're really loaded. But its a very comfortable way to carry a whole ton of stuff.
The messenger designed backpacks, until they're packed, do not obstruct your vision and the good ones have compression straps to 'bulk' down the bag when there is not much stuff in it.