I drank diet soda throughout my entire weight loss and never noticed a problem. (I drank it before weight loss, too, combined with thousands of calories of food!
I still drink about a diet soda a day in maintenance. I drink much less in winter, when I don't want them because I'm cold, and then my summer consumption goes higher.
Some people say they notice an increased craving for sweets when drinking diet soda, but I've always felt they had the opposite effect: Sometimes a Coke Zero is my "dessert" and I can stop there.
I don't fool myself that they're healthy. I know they're expensive and not the best choice. However, it's a compromise I've made with myself - I've changed so much about my lifestyle and food choices, so a diet soda a day is the one vice I still get to keep.