Diet drinks should not be part of your diet!
In addition, there is new evidence that artificial sweeteners disrupt microflora (essential bacteria, microscopic algae and fungi) in your gut. These microflora are very important for your general health. When they are disrupted they drive glucose intolerance which increases your chances of becoming obese and of developing type 2 diabetes.
Sweeteners have been around for a long time. If sweeteners were the wonder substitute for sugar they claim to be there would have been something to show for it, a reduction in weight in the general population, for example. There has been no such reduction – in fact there has been a significant increase.
Beware! Food companies are coming under increasing pressure to reduce the sugar content in sodas. Sugar will be replaced by sweeteners. This might reduce your sugar consumption, but have absolutely no effect on your weight – except that it might increase!