"I like the Peak 1 backpacks with their very adjustable, somewhat flexible frames. I have owned one for several years now. I have found the packs to be stable. Most recently I carried 58 lbs. (usually I go lighter for myself, but with several small kids, I have to carry proportionally more of the load) without any problems.
The packing was on a trail, but it was rocky and steep. When I have gone cross-country, I really haven't given the pack another thought - it has always functioned well. I like the adjustability. Often I will change the strap settings on the second day to move the straps aways from areas that are a little tender from not being used to packing. Also, we recently bought a Peak 1 pack for my oldest son, knowing that the adjustability will allow him to modify the pack fit as he grows.
In summary, I like the Peak 1 backpacks a great deal. I really do think that they combine the best features of external and internal frame packs."