Cinnamon is one of those things that probably won't hurt a diabetic, but may or may not necessarily help, either. My doc said it had to be a specific species of cinnamon, and you couldn't guarantee the supplements you buy in the drugstore are that particular species.
But, he said if it helped, then go for it. He's not against herbal supplements or alternative medicine; he just wants to make sure his patients aren't relying on those things rather than doing what they're supposed to, like staying on a good diet and getting some exercise. He didn’t want them using it as a crutch.