I've got a Lowe Pro fastpack 350 and it will take my camera with 120-400mm lens mounted plus other two lenses, flash batteries,etc in the bottom half of the bag. Leaving the top free for binoculars for 2, or lunch for two. But it won't take the 120-400mm lens unmounted if I have my 17-85 or macro on the body. I need to upgrade to something that will hold more. Also take a monopod strapped to the side.
In reply to Philip: I have a number of camera bags and had looked at all the other options. The f stop seemed to have the features and flexability to allow me to do what i need to do with it . Being able to carry 2 axes , tripod and a snowboard is useful , even though its not what I'd be doing everyday. The down side is they're difficult to get hold of and seem to keep production numbers down or there's just a long waiting list for them .