Did it change at some point? Back when I drank soft drinks I had become a diet soft drink consumer and Coke Zero was a godsend as being at least somewhat like actual Coke.
All I know is that when it came out 15 years ago it was awesome, and at some point probably 5 years ago I realised it tasted like crap and switched to pepsi max.
Nowadays when I do happen to drink some it tastes extremely terrible to me.
Edit: Apparently Coke Zero is no longer sold anywhere except where I live (New Zealand), so I guess nobody else in this forum will be able to check for themselves.
The rest of the world switched to "Coke Zero Sugar" and no longer has Coke Zero. In NZ the new product is called Coke No Sugar and Coke Zero continues to be sold because it was really popular I guess.
I've noticed that Coke Zero bottles that were left out in the sun lose most of their sweetness and taste awful. Maybe there's some photosensitive compound in the sweetener.
I found the same about Diet Coke (Coke Light in some countries). Something changed about 10-12 years ago and it started tasting "chemically" and harsh and I no longer liked it. I switched to Diet Dr. Pepper which I find has a smoother taste. But with the aluminum can shortage, it's become hard to buy in stores.
