I've recently bought one, and have used it with my Fox Adjusta chair which is fairly light. Doesn't need a degree to attach it, but I do have to unclip the bottom of the straps to feed through the deck-chair style legs. When roaming, I don't bother with the chair-clips, the staps hold it in place.
But I do agree about the straps sliding off, I have had trouble with the bag coming off when clambering down steep muddy banks - very entertaining for any witnesses! A chest strap would make all the difference.
Part of the reason for buying this was because it's flat shape sits nicely on the steep sloping pegs on the middle Severn. My normal bag could too easily roll down the bank and I've nearly lost it a couple of times (including phone and camera).
If you think the lightweight chair is heavy, the standard Korum chair is 14lb! Actually 9lb isn't too bad. The Fox Stalker is 12lb. The Chub lo-lite ("super lightweight") is 8lbs. IIRC, my Adjusta chair is 7lb.