Hi Andrew,
i do have the Stubben and love it to bits. for it's size, it is quite weighty, but i can get a fair amount in it. the pockets are all a decent size and, after a good coating of wax, i've never had any issues with water ingress, even in torrential rain and wind. though, because of the front zip, there is a chance water can get in. it is certainly no less waterproof than any other sack i've ever owned.
The seat is very comfortable as is the simple back system. be warned though, the back wouldn't suit a shorter person, Swmbo is 5' and a peanut, and finds the bottom of the sac digs into her bum. i'm 5' 10" and it fits me perfectly.
Hope this helps