I love sweet iron or copper bits because they encourage salvation and lots of horses like the tast so they're more likely to want to be bridled. Some horses also are more eager to push forward to a copper or sweet iron bit.
They always rust and that is where the sweet taste comes from I think. I think it is supposed to go rusty :)
I'm not sure but those sweet iron bits do tend to rust, which can't be good?
My horse has had sweet iron and he certainly enjoyed having it in his mouth, particularly when it hadn't been used for a while as the rust had built up.