Yes, Diet Dr Pepper is more like Regular Dr. Pepper. I've always wondered who came up with that, 'is more like' implies there is something that is 'less like' it somewhere, so are they implying that Diet Dr. Pepper is more like Regular Dr. Pepper than Diet Coke is? or do they mean that Diet Dr. Pepper is more like Regular Dr. Pepper than Diet Coke is like Regular Coke?
Those musings aside, it is reasonably close in taste. It is one of the few diet sodas I will choose to drink.