Saw this at 7/11 and I couldn't resist. It's milk. End of story. Even if it's carbonated.
Sooo... How is it? Unusually delicious! If you've tasted Vanilla Coke, then the comparison is easy, if you haven't... Well, describing the taste would be harder so I suggest you just go try it.
Just a word of caution, this ain't for the faint of heart! The taste is not for everyone. I believe you should have an "adventurous" taste bud to appreciate this.