I actually have, and occasionally use one of these sling packs. I found myself fishing the B'kill in Vermont and it turned hot. I decided to stop by Orvis and ended up picking this pack up.
I keep it in my bag for the occasional day when the vest is too much. I find that I put what I think I may need in the sling from the vest and go on my way. In this application it isn't too heavy on one sholder because it isn't stuffed like the vest, but I still use my vest whenever possible. I see myself using this more in summer months for bass or whatever I can chase while waiting for fall.
"Life is about the journey, not the destination" - Me