to be honest, yes, it is a heavy sack, but not so you'd notice. the seat is very comfy. i've spent several hours in Scotland photographing otters whilst sat on it and not once did i have a sore bum.
and yes, it is quite pricey too, but, compared with some of the other rucksack/seat combos, it is far more comfortable to carry and sit on, the material is very rugged and it is very weather proof. i like the idea of the removable side pockets, so i can take one side off to hang my camera etc.
one thing though, it is quite a long back, so wouldn't suit people of a shorter stature.