Diet soda is the only vice I have left! -- I'll give it up only when they pry my last bottle out of my cold dead fingers. They'll probably also have to tip me out of my office chair and close the spreadsheet I was working on, but that's another matter.
Seriously though, as long as you're not drinking a silly amount and the caffeine doesn't either make you dehydrated or keep you up at night, then why not allow yourself something you like?
The only thing I have a problem with is the marketing ploy around Stevia, it's no more natural once isolated and concentrated than any of the other 'artificial' sweetners, all of which occur naturally and have been isolated and concentrated. As for safe, as others have stated here, all the currently used artificial sweetners have been thoroughly tested, far beyond what would normally be necessary, and none have been proven in any credible studies to be the stuff of evil that the anti-everything crowd say they are. So the only real choice is which aftertaste you prefer.
Compared to when my Dad was diagnosed, and sacharrine was the only option, I think we are very fortunate to have reasonably decent tasting sugar substitutes so that we can still enjoy many of the foods and drinks that others do if we wish.
I think I'm just going to try and slowly get myself off them as it will probably save me a lot of money in the long run lol