I have ridden in jointed irons and hated them. I don't know if ya'll have seen me ride but seriously I went from having a stable leg to having to shorten my stirrups two holes to keep my legs on the horse. They can make your leg very unstable and I do not recommend them at all for dressage.
For jumping I find that they do help my knees, but I far prefer the 1 way swivel irons (http://www.derbyhouse.co.uk/user_res...40122-01_2.jpg
) because they not only look 100 times better, but I find them way more comfortable.
My coach rides with the turny ones, but she doesn't turn them. I think it depends on how long your leg is, I know they would probably help me on a bigger horse, but on my horse I like how my stirrup leathers lay across my boot to help keep my leg more stable. On a bigger horse I would probably want less between me and the horse and I might have some horse-side to lay my leg on. It all depends, but I think they are a nice option to have just incase you're riding something you might need them on.