If you remember in the past Coca Cola used to lace its drinks with cocaine.This was so its customers would become addicted and keep buying there product. Nowadays they use a cocktail of chemicals. The blend of Caffeine and sweeteners ( which I know you said you were not to bothered about ) cause cells in your brain 'apparently' to get a feel good sensation then die. You have loads so not too big a deal but sounds bad, lol. The diet drinks cause fast water retention in the body and leaches calcium from your bones. The calcium in your bloodstream causes your body to mistake this as a source of nutrition and you get cravings.Think that's about it. On the plus side they taste awesome
I think like most things in moderation is ok. A side note a lot of anorexics use diet soda as a meal replacement. This is not good but guess shows works for dieting.
Watch this for laugh on the water retention side effects : Fat to fit in 5 hours
Have fun and good luck with whatever diety thing you are on. Might be wrong on the water retention bit but meh
sure I came across that somewhere lol.