The first bag I bought was a sling back and don't use it, huge waste of money for me.
I think it's best to have your camera out and ready, no matter how fast you get with a sling back, honestly it's still usually too slow to be that effective.
The other reason I don't use it is it's way to easy to make a mistake and not secure your camera or close the bag all the way.
Your camera can easily fall out if not secured and even the most careful person makes mistakes.
Not to mention it kinda helps a pickpocket/thief.
The main advantage of the different packs lowepro makes is they open from the inside part that touches your back when wearing it.
Even if left unzipped all the way, it's still impossible to loose your camera/flash/lenses even if surrounded by pickpockets and thieves.
Also, their pretty well built and comfortable as far as backpacks with heavy loads go, plus they're very protective from bumps, weather, and anything else.