There's a couple different schools of thought on that...
Bottom line it is 0 calories.
But, it is not a source of water. You still need to get your water in for the day.
A recent study, which I am unable to cite, showed that diet coke drinkers actually gained more weight over a period of time than regular coke drinkers. Apparently, diet soda actually increases your appetite.
Maybe it's just that fat asses tend to always drink diet soda along with their cheeseburger deluxe and cheese fries. Who knows.
I don't think there's anything wrong with a couple cans a week at all, I think you're gonna see sub-optimal results if you make it your regular beverage of choice, or even do a can a day. It's putting a lot of junk into your body.
But as a treat when u need to kill a craving, go for it. I'm a diet mountain dew guy myself. hehe.