I think drinking diet soda daily could possibly be an issue, but letting yourself have one as a treat from time to time is totally fine.  I have been drinking it for about 2 years now and I'm maintaining my weight perfectly.  The studies coming out now about it affecting your stomach bacteria and causing weight gain could very well be true, but they're not affecting me.  I have always assumed that diet soda weight gain was horseshit and was simply because fatties decide that since they skipped the 500 calorie soda, now they can have ice cream.  What are you going to do?  Fatties going to fat.