I have made this cake quite often. I used regular and diet sodas. I even used the flavored sparkling water at Wal-Mart since it is sugar free and it wont give you gross colors. Are you located in a high altitude area? Did you use only 12 oz of soda and one regular box of cake mix? Was the soda fresh (i.e. not flat)? Did you bake the cake in an odd shaped pan (usually wont effect things but you never know).
some of my favorite flavors to make were to use the strawberry flavored carbonated water and mix it with the chocolate cake or a strawberry cake mix. Also the cherry Dr Pepper is excellent with the chocolate cake. Kiwi Strawberry carbonated water with the strawberry cake or even the white cake mix is good. ALL the flavors with a white cake is nice (I especially liked the Peach flavored water with the white cake mix). And ginger ale with a spice cake is interesting as well.