I have the Eberlestock J34. I have been training with it since early July with 68 lbs total wt. 2 days / week, about 4 miles each day. That is my only experience with it. I will be using it this October on a mule deer hunt. I think for the price, the J34 is a good pack.
When loaded down, it will have a tendency to feel like it 'sags' just a bit after about 30 to 45 minutes into my hike, which puts more weight on the shoulders and pecs There are definitely better packs out there, but as CoHiCntry stated above, they are significantly more $$$.
It does have a built in rifle scabbard which is nice. The material is very durable and quiet. The pack will haul more than most people can physically carry without risk of injury. I think there are several dealers on line that sell them for somewhere around $325 to $350.
Good Luck