I have smallest Lowepro Slingshot, the AW100. I take it everywhere. Mostly I throw it over my left shoulder. If I'm going to be doing a lot of walking I pull the strap on over my head so that it crosses my chest. I thought it would look dorky that way but it's comfortable so I got over it.
Currently the AW100 has:
Top Compartment: MiniDV Camcorder, an extra MiniDV tape, the camcorder USB cable and the D80 USB cable.
Front Pocket: Dell Pocket PC, Bluetooth USB adapter, USB Thumb Drive, USB memory card reader
Main Compartment: Nikon D80 with 18-135 lens attached and hood reversed, Nikon 70-200 with hood reversed, SB-600 Speedlight, Small bag for extra AA batteries and the base for the Flash, Laminated D80 quick reference guide, extra memory card.
And that's the smallest Slingshot
Jann, I'm not sure what you mean by holding the bag horizontally. I pull the bag under my left arm to the front of my body. This leaves the pack on it's side with the front of the pack perpendicular to the ground. To get out my camera I only have to partially unzip the main compartment. I only unzip the side that is now on top. I don't undo the buckles to completely unzip the main compatment unless I am changing lenses. I can get my camera in and out pretty easily without taking the pack off. I just pull open the velcro into the section with the flash if I need it.
That's one of the larger packs in the picture but that's the position for getting the camera in and out.