I have a Kifaru Scout and I absolutely love it.
I've used lots of packs, but it is hands down the most comfortable one I've ever used. As has been said, most day packs don't really have a proper waist belt. Especially for a freak of nature like me. A waist belt is often times an around the rib cage belt. Hence all of that weight is borne by the shoulders. I'm a big strong guy, but anything over 10 kilos and it starts to become a real drag after 10 kilometers.
With the Scout, all the weight is borne by the hips. I have carried 25 kilos and it feels as though I am carrying a fraction of that weight. Mind you it has a lot to do with a really good suspension system, that you dial in to suit you perfectly. I can actually set it up so that as little as possible of the pack is actually touching my back. That sure is nice on a hot summer day. Unlike a regular back pack.
The other thing that is nice about a lumbar pack is if you do any cycling, it sits down low so as not to obstruct your view behind you.
Running with it is also a pleasant surprise. Unlike a traditional pack that bounces around on your spine, my Scout is on my hips, and doesn't move. I dread having to run with a regular pack on, but with the Scout the suck factor is actualy fairly low.
Oh yeah and the waist belt on the Scout is the best of any pack I have ever used. I can wear it with belt and shoulder straps, just shoulder straps, or just waist belt.
Kifaru rocks!
And DwH, you are correct. Patrick Smith of Kifaru, was the founder of Mountainsmith.
Thomas