We have made a couple of these and they are pretty good. We did one with Diet Seven Up and a yellow cake mix and one with Diet Dr. Pepper and German Chocolate Cake mix. The yellow cake one that we did tasted kind of like lemon cake and I think would have been even better with a lemon cake mix.
We are making these in a bundt pan and taking the cake out of the oven at the lower time of the cooking time range listed on the box. Also this link says one can of diet drink. Wherever my wife found this recipe to start with said 20 ounces of drink, which is actually what we have used. http://www.ehow.com/how_5331843_make-watchers-diet-soda-cake.html