it really depends what you're looking for. when hiking through the canyons of Utah and Arizona, a real "backpack" with nothing but camera and eqpt in it was fine and I got the Tamrac Expedition 4. But you had to stop, take it off your shoulders, put it on something flat, open it up, get out your gear, zip it up again, blah, blah... I now only use it when I'm traveling by air and want a camera bag that I can carry on both shoulders, freeing up my hands. When I get to my destination I completely unload it...
Now that I do a lot of shorter day trips (walking around in NYC, etc.) and carry a 17" MacBook Pro often, I got their new Adventure 9 and I LOVE IT. It carries a 10D with a big Sigma 35 - 70 2.8 lens on, has spaces for two more lenses and a flash and stuff, and is accessed through a back panel. It also had an upper compartment for junk, food, a jacket, whatever other stuff you need. It comes in smaller sizes and a somewhat cheaper version (Aero), but it's a great combination.
They also make some "slinger" bags that emphasize quick access in a quasi-backpack form, but I think they're just glorified shoulder bags and so I still use my old canvas Domke bag for that...
Good luck!
Adam