I have used these packs exclusively for 3 years and hundreds of miles with my two border collies. Kye is a 34lb female and Cody is a 40lb male.
When first buying these packs I bought the small for Kye and the medium for Cody, according to the measurements and weight guidelines on the website. Despite this the medium pack really was too large for Cody and I had it cinched down as tight as possible to make it fit. The panniers themselves were massive on him. Recently I replaced his medium pack with a small and it fits much better and has smaller panniers.
What I love about these packs is how they fit and carry on each of my dogs. The weight is carried over their shoulder and is not suspended from their back like other bags. This creates less fatigue and rubbing.
The fully-adjustable 5-point harness of these packs means that they do well on any shape or size of dog (assuming you get generally the right size), and as long as they are adjusted correctly I have seen absolutely zero chaffing, rubbing or hair wear during or after our backpacking trips. The key to this is not having the harness too tight; it needs to keep the pack on the dog NOT keep the pack in place on the dog. Keeping the pack in place is achieved by correct placement and balance of the pack’s contents, much like the panniers on a pack horse. I have seen several dogs with packs hanging to one side…this is uncomfortable for the dog and causes chaffing and fatigue.
The panniers themselves, on a size medium, have the capacity to carry about 3-4 days worth of regular dog food (about 2 cups per meal) on each side. I usually only have my dogs carry two meals on each side due to weight more than capacity.
The fabric of these packs has held up extremely well. My dogs generally stay on trail with me but they have been through boulder fields and up talus trails, through scrub, under and over trees, between rock faces and more. The only wear, after 3 years of use, is along the very bottom of the panniers where have often rubbed against rocks, trunks and tree limbs as the dogs have climbed over or around them. Even snagging on large branches has not affected the fabric.
These packs also feature a grab handle (useful when Kye almost fell between two boulders on Jackass Pass in Wyoming) and a leash D-ring if you don’t want to use their collar (which every dog should be wearing on the trail). The positive aspect about the leash connection on the pack is that a leash is less-likely to get wrapped beneath the panniers than if using their collar.
All in all these are great packs, they fit well and my dogs love them. Highly recommended. The only packs that I have seen that are comparable in quality and fit are Groundbird Gear Dog Packs but they have a long wait period and are much more expensive. Ruffwear dog packs are readily available in REI and at online retailers.