My biggest issue with diet soda's besides the fact that they all have that same nasty tainted taste, is that you are putting artificial sweeteners in your body.
Over the years countless studies have been done to show that putting anything artificial in your body is just not a great idea. The idea that a beverage can be healthier for you simply because it has a substitute for sugar in it is just silly to me.
Aspertame is one of the biggest things that they used to use in sugar free sodas and foods. They have started to cut back on those now because of issues they found with them. Olestra was another synthetic chemical they were putting in food as a substitute for something else.
More recently, they have even started to tell people to go back to eating full fat yogurts, butters, and other foods. To make those low-fat foods reduced fat they had to add a bunch of sugars or artificial sweeteners which are just as bad for you.
I have actually made an effort over the past 15 years of my life to cut out soda altogether (unless it is mixed with a little rum). Are you immediately going to see the negative results of diet soda? Probably not. Is it something that could kill you slowly over time via other issues or complications? Possibly, but there are also many other things that could do the same.
Eating a pound of bacon every day could be just as bad as drinking 40oz of diet soda every day. Is one better because it is more natural than the other?