I think the occasional diet soda can well be a good thing...adds some variety and flavor and kind of provide a "break" to the rigors of "healthy eating". I am very adverse to caffine, so I always make sure if I have one it is caffine free.
I used to be against it, but after watching my wife diet and get ready for competition(that she ended up backing out of by the way at the very end), she drank diet soda daily on her diet, and was lean as hell and looked great. (she should have followed through with it).
-------------- Maximus from Gladiator....Strength and Honor!