I've sat in a BA crew jumpseat for about 9 hours and it was horrible! I wouldn't wish it on anyone! The seat has minimal padding (so it can flip up), the harness, which had to be fastened when the seatbelt sign went on, which it did, was a full over the shoulder effort, very uncomfy, and you are absolutely bolt upright!

I ended up in the seat because the lady of extreme size was taking up her seat, half her husband's seat and my seat - I couldn't put the armrest down, even slightly! In retrospect, with more miles under my belt, I should have refused to allow it and had her offloaded - there wasn't a spare seat on the plane in any class, and the next flight out was something like 3 days later!