I've been using the ThinkTank Airport Antidote, which fits a 15" laptop on the outside. I bought it for traveling, and specifically for riding a bike while carrying camera gear and a laptop.
I used to think having the laptop against your back was the best because you'd be carrying the heaviest item closest to your center of gravity, and it would keep the laptop more protected. Now I'm sold on keeping the laptop on the outside of the bag. Camera gear is much heavier than a laptop, so that should be against your center of gravity. Camera gear is also generally broken up into components, which allows a little bit of flex in the pack to conform to your back curvature, especially while riding a bike. Lastly, if it's a camera bag with the ability to carry a laptop, chances are that it will always be carrying camera gear, but only sometimes be carrying a laptop, so I'd rather have the empty space on the outside, rather than against my back.
As far as the gear you can fit... I recently packed two bodies, a 70-200 f/4 IS, an 85 1.8, a 17-55 IS, a 530EX II, all relevant chargers and spare batteries, some filters, and a sandwich, and still had some extra room.
