I used to drink a lot of diet pepsi thinking it was not bad for weightloss at all since there's no sugar, no sodium, etc in it. I had gotten serious about dieting and exercising, yet I was not seeing the results I wanted. I quit drinking diet soda completely and the weight immediately fell off. I started drinking it again and the weight stopped coming off again! As soon as I gave it up for good, I had no problem.
Of course it's better than sugary soda but my trainer buddy said in alot of people, their body reacts to fake sugar just like regular sugar so you hold on to the weight, plus carbonation leads to lots of bloat!