From Will Rietveld's review in my link above:
The short answer is the Kalais is one of the best backpacks I have ever tested, and I have tested a lot of packs. As with the Aquilo, Elemental Horizons got (almost)(the thing he was not keen on was the water bottle pockets on the side of the pack which they fixed after Will's review) everything right the first time, and the Kalais is another winner.
The Kalais carried a 25 pound load surprisingly well. I noticed the weight, but the pack transferred all of the weight to my hips. It was at this point that I discovered that the Kalais’ four-pull hipbelt tightening system makes a big difference for carrying heavier loads. By separately tightening the upper and lower pulls, the top and bottom of the hipbelt can be tightened separately, which conforms the hipbelt to the hips and prevents it form slipping under heavy loads.
Elemental Horizons uses a removable contoured aluminum stay that is similar to the one used by Gossamer Gear and Six Moon Designs, but its integration into the pack is much better. Rather than a simple straight sleeve on the inside of the backpanel, the stay is more solidly anchored at the top and bottom of the pack, so it transfers weight to the hipbelt better, as my testing results indicate.
I own the Gorilla,the HMG Windrider and the ULA Circuit and I have to say that I agree with Will,it has become my favorite pack and carries better than my others.