Why I have a camera slingbag but you probably shouldn't
Camera slingbags are inherently a compromise. Backpacks (and some belt systems and the like) provide better support, while shoulder bags and belt systems provide faster access to gear. This compromise nature is why I think you probably shouldn't get a slingbag since you can do better on either aspect and in the long run I think the compromises inherent in a slingbag will prove irritating.
(Especially I would not get a big slingbag because slingbags just don't provide enough support for carrying a relatively heavy load.)
Why I have a slingbag, and specifically why I have a Lowepro AW-series slingbag, is that I am an impatient bicyclist. As a bicyclist I need to carry my camera in some way that keeps it both stable and out of the way; this rules out both belt systems and anything like a plain shoulder bag. As an impatient bicyclist I want my camera to be relatively quickly accessible so there is not a big time-consuming production involved in stopping to take a picture; this rules out backpacks. So I'm left with the Lowepro slingbag; it's stable enough to stay in place even with relatively aggressive bicycling and being able to just unhook the stabilization strap and sling the bag around keeps the camera accessible enough to make me happy when I stop to take pictures. I live with the relative lack of support (which I can definitely feel on long days with relatively heavy loads) and the relative lack of fast access because I need the particular combination in the middle.
(Why the Lowepro specifically? Because it has a second stabilization strap that holds the slingbag firmly in place when it's clipped on to the main strap. I've been unable to find any other slingbag with such a stabilization strap, although I haven't looked everywhere.)
PS: in some ways I'd be better served by a handlebar bag that was big enough for my camera (and lens), padded enough so I could trust it to not rattle the camera too much, and detachable so I could take it with me when I get off the bike. Unfortunately you really want drop handlebars to create the cable-free space for such a relatively large handlebar bag and, well, I don't have them on my current bike.
(Update: it turns out that I'm wrong about the Lowepro being the only slingbag with a stabilization strap; they're actually reasonably common if you look around and read specifications in detail. Due to not having seen or played with any of the alternatives in person, I have no opinions on the relative merits of my Lowepro versus the alternatives.)