I've wasted so much money on cheap backpacks that couldn't hold all my gear and that had problems the first time they were stressed.
My 2 cameras plus 3 extra lenses fit easily in the 400 AWIII. Its depth allows me to arrange items in multiple ways. I can easily leave the spare camera in the car and use the top space for clothing or odd items.
The backpack fits comfortably with the weight distributed well at my hips instead of my shoulder. I love that it opens from the back so I don't have to put a wet or dirty backpack on my back after changing lenses or pulling out filters.
I am looking forward to see how it holds up to a trip to Death Valley in March.