I've personally been considering picking up one of the Stoney creek "Magnum Waist bags" for two reasons. Firstly, I do alot of walk about spotlighting and carry a 7 amp battery in an average bum-bag and I'm finding that the strap is just cutting into me as it's not designed for the weight. Secondly and for me, more importantly, I don't like shooting with a back-pack. I find the shoulder strap(s) displace the position my rifle(s) take against my shoulder. To me it feels very uncomfortable and it's very hard to get used to. That is why I opted for the normal bum bag in the first place, but the normal (unpadded) strap just doesn't have the support I'm chasing. The Stoney Creek one has a large/wide padded strap (it looks like a kidney belt) and has quite alot of room, almost replacing the need for a small backpack/day-pack.
Just my two cents,
Pete. :D