I am (as the username suggests 
) a diet coke addict. Have been for years.
Went through a period of giving up entirely (as in I had none for five/six months) when going through secondary infertility -was trying various alternative therapies, sugar and caffeine free diet etc in my attempt to get pregnant.
I didn't fall pregnant. Just felt stressed and miserable than I'd done before. So rather than feeling cleaner and healthier I just felt worse for the 'healthy eating' approach. (I was, however, lucky enough to have two more DC - the first via IVF, coupled with a defiantly normal and non-alternative diet, and the second a natural pregnancy when drinking diet coke, drinking wine, eating sugar etc as normal...
I cut it right down when pregnant, and drank reduced amounts daily during my six-month bf stints after each baby. And had very calm, chilled out, babies who never seemed to react to the caffeine and slept beautifully (oldest two did 7-7 at night with a dream feed from eight weeks old, number three wasn't quite that good but still slept well compared to your average baby). That's not to say it doesn't have an effect on some babies btw - I am sure it does- but mine seemed immune to any ill effects from anything apart from Brussels sprouts .
I would like to reduce my intake though (average is four cans a day, would like to cut this down). Like others on this thread, I drink no tea/coffee so diet coke is my only caffeine hit which is how I justify it to myself. It doesn't seem to affect my moods or sleep, but it costs me a bloody fortune.
Problem is it is so nice...and it just makes me feel happy. That sounds pathetic doesn't it. But then again I figure, as addictions go it could be a lot worse.