Depends on who you ask... There are many who feel that there are metabolic problems caused by most of the artificial sweeteners, such as aspartame, sucralose, sacchyrine, etc.There are many who feel that they are safe in moderate quantities. I have been avoiding them for the last 7-8 months, in part as an 'experiment' to see if that makes much of a difference for me, but also because, though I miss diet soda, I otherwise find I don't really need them.
There are a few "sugar-free" sweeteners that are considered "safe" by most professionals. These are Stevia, Monk fruit extract, and erythritol. The first two are sweet plant extracts that have no impact on blood glucose for most people, the latter is a sugar alcohol they most people do well with. Some people find that Stevia has a bitter aftertaste, but the only way to know that is to try it. BE careful with Stevia products - many of them are packaged as a mixture of sugar and stevia -- read the labels!