I had my first Coke Zero yesterday, and hadn't been clued in ahead of time about what it was supposed to taste like. I was suprised to find out that it tasted exactly like regular Coke to me. It doesn't quite taste exactly like regular Coke once it warms up though.
I saw Coke Zero at the store and couldn't figure what was the difference between it and Diet Coke. I figured it was a marketing tactic to get people who normally avoid diet sodas to try it. Personally, I haven't tried it. I do drink both Pepsi and Coke, but I also drink Diet Coke, especially the caffeine free type if I know I going to sleep soon. I don't want sugar or caffeine in me when I'm trying to sleep. I avoid Diet Pepsi because it's gross.
I drink Diet Coke and recently tried Diet Coke with Splenda. Thought it would be better, but it was vile. To some people, it probably tastes more like regular Coke, but to me, it tasted way too sweet, like chugging Coke syrup. It might be pretty good with half the sweetener, but people are so used to getting too much sugar and too much salt, if you dial down either one, they complain there's no flavor.