I was reading up a bit on Stevia the other day. Supposedly it has no negative effects on insulin and in fact may improve insulin sensitivity, though I don't think that has been conclusively proven. Problem is there was some controversy in the US in the early 90s as to whether or not certain compounds in it are carcinogenic. Apparently there have been foreign studies showing this to be false, but due to politics and potentially a different opinion the US still has it banned as a food additive, but not as a supplement.
Whether that makes it safe to use or not.... I would need to read up more on, but it seems to have the potential to be a pretty healthy sweetener... as long as it doesn't cause cancer