From what I've read, the jury is still kind of out on it.
The main problem with artificial sweeteners and whatnot is (again from what I've read) that some people react to them like they would sugar: with an increased insulin level.
As for the long term effects of it, only time will tell. We're about to see the first generation of people that have had artificial sweeteners most of their lives reach middle age and heading towards retirement. We should know more in a few years whether or not it causes cancer or whatever it is people say about anything unnatural.