I changed from regular Diet Coke to the caffeine free variety after I started Atkins. I found that caffeine was causing me to get quite hungry. This doesn't happen to everyone, but is common enough, I think. Recently I've cut back on it and started drinking DietRite drinks: their cola and orange are quite good, and use Splenda rather than aspartame. I don't think anyone really knows which artificial sweeteners are the best, or the worst. (If you search the web, pretty much every substance known to man is both condemned and lauded, depending on whose site it is.)

Of course, there's nothing wrong with weaning yourself off diet drinks, but it isn't necessary to do so to succeed with a low carb diet.