A sling bag is great if you keep it small and light. A larger bag invites overpacking which is not good one one strap. I have a Maxpedition Kodiak Gearslinger which I pressed into service as a laptop & book bag for college and it was miserable. It's a good sized pack, maybe 25 liters, with a single strap that's fairly thin for the size of the pack. Loaded up with a computer and books for three or four classes it was excruciating to carry.

Many years later I bought a really cool sling bag from Hazard 4 called the Rocket pack. I loved the looks and the format (a sling bag tall and narrow with zippered bottle pouches on either side). Very cool on paper but in practice not as good as I'd hoped. It's around 20 liters with one strap but the strap is very very wide and well padded. Much more comfortable than the Kodiac but the problems still remain, more of balance than comfort. A sling pack doesn't really carry as well as a pack with two straps (and perhaps a waist belt). To carry in the car it works very well, and as a pack that will mostly ride along and maybe be carried from the car to a site or even carried a ways on level ground it's okay. But it fails as a technical pack and doesn't carry well if you carry it on a hike.

To me the "killer app" for a sling bag is as urban EDC or even a wilderness PSK if kept compact. My latest experiments have been moving my large sling PSK contents to a Mystery Ranch 2DAP and working up a smaller PSK in a Maxpedition Jumbo Versipak or GTG bag.