Maybe. I doubt most consumers of sugar free soda are drinking more than 4 (which is already a lot). I have to imagine that, like most things, most people consume them in moderation and have no ill effects.

That said, I have to imagine if you go from drinking ten sugared sodas a day to ten diet sodas a day, your life will change in a very positive way. That would be removing 1500 calories of pure sugar from your diet and that's gotta change people's lives.