Never used the Sling-O-Matic but as I like to take a camera with me whenever I go out of the house, my day-to-day bag is a Domke shoulder bag. It fits a gripped body and two large lenses quite comfortably if none of the lenses are attached. It IS possible to carry an a gripped body with a lens attached but then it starts to get a bit too wide for my taste. It also has a 13" laptop compartment which usually holds an iPad in my case. Also, as far as discreet goes, you can't go more discreet than this bag, IMO. It has a nice brown color and coated with wax so it pretty much looks like a nicely weathered run of the mill messenger bag. You can also remove the inner padding and use it as an ordinary shoulder bag if you wish. Personally I am very happy with it.
As far as drawbacks go, besides the gripped body+lens issue, it only has one zippered compartment. A large compartment, but still I'd like to have a few more, even if smaller, for better organization.
Oh, the model is F-832