Been using an old Timbuktu bag for maybe five years now, it's great in that it's very expandable in shape. Carrys medium weight okay (you have to do the strap tighter and let it ride up on your back, rather than hang off your shoulder).
Pros compared to a backpack:
It's a fair bit cooler in the heat as it lets more of my back breathe.
Super fast access
Easier to pack than a toploading pack
Cons:
For serious weight, a backpack is better with dual shoulder straps
I've tried Chrome bags, which look nicer in terms of design, but their basic bag design is annoying- the top opening is narrower than the bag overall, so you can't overstuff them like with Timbuktu bags. And the chrome quick release is quite hefty.
Messengery bags are a little like keyboards, in that you can geek out and get really obscure stuff. If you look deeply enough about it, there's a bit of a wannabe bike messenger/hipster cultural scene that obsesses over getting the most obscure bags possible. If I were to dive off the deep end on messenger bags, I'd look at something like this:
http://www.bagaboo.hu/ Custom made in Hungary, with some pretty interesting options.
