The LowePro Photo Trekker series is pretty serious, with a lot of the same tech as in their high end mountaineering packs (adjustable back harness, for example). Good fit is the key, same as the high quality waist belt. Check out ThinkTank too.
A good photo backpack should have a lot of the same features/fit/comfort as a hiking/mountaineering/alpine backpack. I do a lot of rock climbing/hiking/backpacking, so I have become very picky with bags. =)
The Trekker's are $200+, but the price is pretty comparable to other quality backpacks that size. Your back won't regret you dishing out the money on it, that's for sure!
I have used mine heavily for 5-7 years at conventions, cities, deserts, mountain ranges, by the ocean, in the forest... the list goes on. Make sure to size the bag properly to your body and strength. If you get a massive bag and are too weak, it'll destroy you.
Lately I have switched to a roller bag, which is more expensive and less versatile than my backpack.
Easier on the body once you get a decent amount of gear, provided you are in areas with good accessibility.