My DH lost weight by cutting out soda, but he was downing like at least 20 cans a week. Yikes! He had horrible headaches and cravings going cold turkey so I told him to filter in a soda every now and then to help with that. That was last fall. We drink Diet now, usually between 1-5 cans total each sometimes non at all. Diet soda alone wont help in the calorie department, but it does still have sodium and contributes to water weight gain if you drink a lot of it.
Honestly though I think its good that you have decided to get of the soda all together. While 1 can a day wont kill you, its not really "good" for you either. Plus starting the day off with healthy choices just sets you up for success to stay OP the rest of the day.