I use a Lowepro Omni Trekker. It holds my Bronica body, prism and 80mm lens attached, four other lenses, a spare back, meter, filters, film and all the other usual suspects. It fits snugly inside a Pelican 1550 so I can check my gear when I travel, though I've never been stopped from carrying the Omni-Trekker on.

I don't like backpacks at all, so I've cut off all the straps that make it dual-purpose, and I've replaced the main strap with an Op-Tech . The bag has two major advantages as far as I'm concerned. First everything is stored on one level- there's no lifting stuff out to get at something else which I think is an enormous plus. Then there's the fact that its about 2lb lighter than the Magnum AW I used to use but seems to protect things well enough. A combination of lighter weight and slim configuration means the weight is all close to your body and easier to carry.

They're about 100 mail order in the UK- I think about $130 or so in the USA.