As I understand it it does not affect blood sugar levels. That is the good part. The bad part is that it still effects insulin levels because as you taste the sweet taste the sensors in your mouth send a signal to increase insulin because of the anticipation that carbs are going to be ingested (they are not, but the signal is still there). So while it does not raise blood sugar it can raise insulin levels.

Insulin is a fat storage hormone (among many other things it does). So by that theory the artificial sweetener may still cause fat storage when a person does not really want it. So it may be implicated in weight gain or lack of weight loss if trying to lose weight.

https://www.youtube.com/results?search_query=dr+ken+berry+artificial+sweetners

John