I use the
Lowepro FastPack 250, in a sling like setup. I use only one strap, and sling it over my left shoulder. Gives you quick access to the camera, and laptop, though that is more cumbersome, and I usually just set the bag down to get the laptop.
It holds my 400d, with grip, and this
Sigma 24-60 with hood attached. Also a canon 55-250 IS lens, a 550ex flash, cleaning kit, caps etc in the camera area at the bottom. In the top section it carries my IT tool kit, some CDs etc, and laptop accessories. And my MBP 15" in the laptop sleeve section. When I have everything in it, its heavy. If I do carry everything in it, I use both straps. I am glad I bought it, and it has worked very well for what I need, but the left shoulder strap, where the lowepro logo is, has ripped the logo on one side. I am sure its because of the weight I have in it, and using only one strap.