There are umpteen threads on this topic, and they all spur heated debate. At the end of the day, NO, it's not that great for you. Just like a million other things. Will it affect your weight loss? Only you will know.
If you're looking for a slightly healthier option, or you find you just need the bubbles, look into a soda stream machine and carbonate the water and add berries or lemon.
Personally I had very bad experiences with diet soda, and it took me a lot of time to quit regular soda. My teeth and my gut and the scale have all thanked me.