Juries still out in this one. Artficial sweetners as you say have no calories so in themselves aren't going to make you gain weight however I have heard some thories that they can mess up your metabolism/ confuse your body.
Against aspartame (the main sweetener in most diet drinks)
Study: Artificial sweetener may disrupt body's ability to count calories Aspartame: What You Don't Know Can Hurt You
For aspartame
Aspartame Information Center - Aspartame Myths and Urban Legends Calorie Control Council | Low-Calorie Sweetener Benefits Q & A
This is definately a for every expert there is an equal and opposite expert situation.
I am a biochemist but I don't know where I stand on this debate. Both sides have compelling and reasonable arguments. Only time will tell I think. The point here is do you want to be the guinea pig? Everything is fine in moderation, I myself consume sweetners but mainly due to lazyness - they are getting very hard to avoid!
You should have a look at these web pages and then do what you want. Your guess is as good as mine.
Here's a tip about sources and research articles - always check who funded it? Do they have ulterior motives ie selling a diet drink?