Hatte auf einer Städtetour die D200/17-55/Zubehör in einer Schultertasche... nach 2 Stunden tat mir die Schulter weh. Nee... Schultertasche verwende ich nur um die DSLR vor Ort zu bringen, aber nicht auf stundenlangen Wanderungen.
Dem Kommentar von Thom H. ist nichts beizufügen..... er hat leider recht!
Backpacks work better than shoulder and other bags as the equipment list gets longer. It's simply the comfort thing. A well adjusted backpack that is fit to your body can carry a quarter of your body weight without slowing you down or making you uncomfortable. I routinely drag as much as 40 pounds over 12 miles without having sore shoulders, back, or neck. Shoulder bags are like golf bags used to be: they put asymmetrical force on your body, and ultimately that means aches and pains. If you're out for shorter times with only an extra lens and a flash, sure, it might work fine to use a good shoulder bag, but as those things get added to, beware the dreaded strap fatigue. The final alternative, belt bags, has the advantage of carrying the weight where your carrying capacity is highest (hips), but the disadvantage of being awkward for carrying long distances (a telephoto lens case on a hip belt system tends to either stick out too much or preclude leg movement). Another comment: a really good, accessible backpack that is well organized (by you) can net you anything you need in a fast frenzy of motion: pack off, unzipped, item grabbed, rezipped, pack on.