it is worse than normal soda but I have yet to read any conclusive studies that support this. The only thing I've consistently read is that the artificial sweeteners can possibly lead to increased appetite to eat sugary foods but that it doesn't actually lead to weight gain on its own. Until I read something conclusive, I'm going to continue enjoying a diet Mt. Dew after all my long runs.

This is a really good article I read on the subject a while back:

http://www.theatlantic.com/health/archive/2014/08/artificial-sweeteners-probably-okay/378937/