In reply to Blue Yonder: It is great for backpacking with around 15kg. In fact for total pack weights in that range I would say it is almost ideal.
They are also perfectly useable for higher weights. 19kg is normal for me for 4 day trips when I'm working and that is fine although not ideal.
However at weights of 23kg+ its limitations such as the lack of padding on the relatively narrow waist belt become very apparent and it is much more uncomfortable than specific backpacking or expedition packs.
However, I replaced my Black Ice with an Alpine 50 (same load carrying comments apply) and I think the later is marginally better for backpacking use.