The only thing that outnumbers the now-unused camera bags I own is the lineup of tripods that I tried to like before getting my current one. All of the bags had been either shoulder bags or backpacks, and one vest-strap plus lens cases. The shoulder bags were the most convenient, but took their toll on my shoulder and back over the years. Backpacks are the most comfortable but, of course, not made for quick, on the fly lens changes.
I've been using the LowePro slingshot 300 for all-day walks since the bag came out, I think, 2-3 years ago. I liked its design compared to the others that were available at the time. I found that if I am careful about adjusting the way it hangs, and keeping the waistbelt stabilizer buckled, I can go all day with no problems. I'm 68 years old, and haven't looked back at my old bags once.
What I keep in the bag: Canon 1DMkII with a RRS L-plate, a full-frame fisheye, 16-35mm lens, 28-135, and a 100-400. The bag is tall enough to fit the camera with the 100-400 mounted, with the lens hood reversed. I also carry an extra battery, 2 hardpacks of 4 CF cards each, a Rocket rubber blower, wallet of filters, remote cable, several microfiber lens cloths and a beat-up fishing hat for sun protection. When shooting handheld series for HDR I slide the pack to my front and rest my elbows on it for stability. I think it is one of the best designed bags made.