I have both the 5L and 10L original Peak Design slings. Of the two, I much prefer the smaller 5L version. The 10L is OK, but at that size and potential weight, I would prefer a normal shoulder bag worn cross-body.
My normal use case for the 5L is when I am out using a TLR - plenty of room for the camera, camera strap, film, filters, light meter. Once I am using the camera (which being a TLR, goes round my neck), I can swing the (now practically weightless) sling bag round behind me and almost forget it's there. Mine looks a bit beat up now, but it has seen a LOT of use and still works perfectly.