I have the J104 and the X1.
I use the X1 for university (for the time being), and the J104 for everything else.
I had 80-90 lbs in the J104 and walked for 2-3 miles, through swamp, gullys, briars, brambles and bushes where a rabbit wouldn't go. I felt fine and I'm not a big guy(I wrestle at 54 kgs, do the math).
I've never loaded the X1 up a with a whole lot, but it seems like it will handle the weight as well as the J104.
They both feel like they will last a long time, and for some hunting having your hands free is a real asset.
I also strapped my climbing tree stand to the J104, it was FAR better than carrying the tree stand on my back.
If you need a huge pack you can get the J104/107 with a zip in duffle bag, it gives you a crazy amount of space to haul your crap around.
Overall I am very pleased with Eberlestocks.