I bought a saddle stool and I loved it for my back but I've had a hip replaced and it pressed uncomfortably on that hip-bone so I had to stop using it. Now I'm using a balance ball - it hasn't been that long but so far I like it a lot.
I have a terrible habit of scrunching up my shoulders and hunching over when I sew, even though I've carefully set up my station to be ergonomic if I sat properly. So now I'm trying seats that force me to sit up straight(er) in order to balance, and it does seem to be helping me not hunch up as much. Now when I hunch up I have to use my stomach muscles a lot more to stay upright, so I guess if it's not correcting the problem, it's at least helping me exercise some pretty much unused muscles, LOL.