In my opinion, it is all about quantity and portion size. A cup of coffee a day with a non-sugar sweetener isn't going to cause you much harm (as long as you don't have an allergy). But using a non-sugar sweetener in place of regular sugar when you have a huge sugar habit isn't going to be good for you, nor is it going to solve the fundamental problem of an overly active sweet tooth.
So if you are inclined to put 3 teaspoons of sugar in your coffee, drink 5 sodas a day, and can't resist desert after lunch and dinner, simply replacing sugar with any of the artificial sweeteners isn't going to do your health any good - for many reasons including prolonging the sweets habit.
It is really hard to break the sugar habit - but it can be done. And once you do, you will have crossed a hugely important milestone.