I have had around 10kg in mine with no swing what so ever, but you have to be aware that it gets very rear heavy. I've used it to take my mac with shoes etc underneath, a file on top and a jumper packed over the top. I think they are amazing bits of kit, very liberating being off the back.
Thanks. How do you find it deals with the weight? I'd be afraid of it swinging side to side as my pack is ~7.5kg