I've done a couple of longer rides this year carrying a backpack, 3 centuries etc without issue.. Karrimor Sabre 30 packed with normal clothes, pump, tools and a bottle or two of water, maybe 4-5kg altogether and that worked very well. I wouldn't want to carry much more though. It does cover the back but has very good damping and ventilation and the straps are the most comfortable I've ever had. Wouldn't even try with a "normal" bag like an Eastpak or somesuch.

It looks a lot bigger than it really is in this pic