After several trips to the sandboxes, I am still using the Mark 1, Mod 1 GI issue sling. It does everything well, not as perfect as a "purpose-designed" one, but it works for everything. If you are like me, and favor utility over 'gee-whizz,' there is nothing wrong with the GI issue sling. I have rigged it as a one-point, two-point, three-point (thank you 550 cord), a hasty and a loop sling. It is durable, easy to take care of, and works.
Sgt Ski, USMC