I use a Lowepro Super Trekker AW. I've had it for some time, and the new ones may have a better suspension, but mine doesn't carry as well as my Dana Design pack. The problem with top loading packs is that it takes more time to get your gear out. Seconds count when the light is changing. If you're moving around between compositions, the photo backpacks save even more time.

When I used my Dana pack, I bought a couple of plastic containers; one for the camera and some accessories and the other for the lenses. I lined them with open cell foam. That kept things relatively organized and meant that I only had to remove two items from my pack when setting up. The advantages to using a conventional backpack are 1) cost because you may already have a pack, 2) you likely will have more space for a parka and other personal gear, and 3) better suspension.

Photo backpacks are much more expensive than high end backpacks. Justification of the cost differential is a personal thing.