Executive summary - is it worth it? Yes.
Aarn packs have a number of innovative features and my mod does not create an Aarn pack, just the front loading balance pockets. Making them work with a non-Aarn pack is pretty easy and well worthwhile. The pockets are great for cameras and lenses as well as food and drink.The front loading reduces shoulder and back strain (a lot). I realise I am not the first to do this but I liked the result and others have said they wished Aarn made pockets they could add to their pack. They do.
I was looking for a better way to carry my DSLR with a pack. I had been clipping a dry bag round my neck which sorta worked but looked pretty odd. Aarn make special pockets designed for cameras with extra padding and pockets for lenses but these were not available so I bought a pair of the Sport pockets from the Australian distributor (excellent service). The pockets attach at 3 points - a clip to the upper shoulder strap where the chest strap attaches, an elastic loop laterally to somewhere on the waist harness near your hip and finally a metal strip that goes the full length of the pocket and carries all the weight of the pocket down to the waist belt - there is no shoulder loading. The metal strip slides and clips into small pockets either side of the main buckle. You can buy an Aarn buckle and swap the one on your pack however the Aarn buckle does not have any friction to hold the waist belt tight - there are buckles at the hip to do that. Ultimately I added modified triglide buckles in a similar place - with the pockets full it is easier to adjust waist belt tension at the side and the WE pack setup suits this arrangement. Other solutions are to remove the "wings" from the Aarn buckle and rivet them to your packs original buckle or to use a friction device such as my modified triglide to hold the belt tight. The exact solution might vary between packs . I can post pictures of my solution if anyone is interested. I added heavy duty press studs to hold the metal strips in place similar to Aarn but tighter and added a cable tie to tighten up the waist belt pockets and hold the metal strips more firmly - previoulsly they could slip out when putting the pack on.
As supplied the metal strips can produce pressure above the pubic bone when bending or climbing steeply - they need the bottom end bending about 30 degrees froward which completely fixed the problem for me. I think I have the waist belt a centimetre or so higher than I used to with a very tight waist belt to get near 100% waist loading and unweighted mobile shoulders. My experience is that you should aim to get about a third of the pack weight in the pockets to get the balance effect - they lean forward a little away from the body so have a bit of a lever arm and dont need to be 50%. So you need to load them with dense stuff. Some seem to have problems putting on a pack with balance pockets - I can only imagine these are the folk who grab one shoulder strap and swing the pack violently in a wide arc until it collides with the other shoulder or the bloke standing next to them. If you rest the pack on your knee and slide one arm in there is no drama at all. But when lifting a pack with one hand by its carrying loop the pockets do flop about due to the extra weight. I would always take them off and put them inside the main pack on buses and planes. Fortunately you can take them off in 30 seconds or so.
The pockets are not cheap at around $100 but there is a lot of workmanship in them - lots of hooks and straps and pouches to increase their functionality - they even convert to a pretty good small daypack. They include a custom light drybag. I have walked about 50km with my new setup with around 20kg total pack weight and they are fantastic. If you unload the front pockets and put it in the main pack you suddenly feel what it used to be like. 20kg is a big pack for me but it only feels that way as I put it on and take it off. I suspect that a generation or two ago the average Aussie had a lot more upper body strength working in a more physical job, splitting firewood and using a car a lot less. The packs I remember from the 70's with two leather shoulder straps and a thin strap for a waist belt that carried no load were better suited to that generation. Maybe they were simply more stoic. Whatever the reason it is my experience the more weight I can carry on the waist the better. The more upright the posture the less low back pain. The more free the shoulders the more normal the gait. If I was in the market for a new pack I would buy an Aarn but grafting pockets on your current pack allows you to try out one feature of his packs at moderate cost. As a DSLR solution it is great.
Chris