I haven't used it, but was considering it for our upcoming pool deck. The reviews on Amazon seem good, and when I asked a question there someone responded with their positive experience of 3 years.
There is a negative review on YouTube if you haven't found it already, with a guy who used the product in a similar situation to yours. You might want to look into it. I don't know the veracity of his claims, but the company does seem to admit that the product can't reduce temps as much when applied over an existing product layer (as opposed to over regular concrete) because the first product acts as a barrier to the "heat sink" action. They do say that it will maintain at least the same temperature as the original Kool deck surface. Cosmetically, I'm not sure how it works over something like Kool. That's where this guy had a gripe, but the company claimed he didn't prepare the deck properly or something.
I too am interested in firsthand experience with the Cool! product. This is just what I've learned from my own research so far. I hope it helps.