I haven't mixed the coffee batch with the caramel batch yet, but the one with the added coffee extract is very good, but very young. I added the coffee extract 8 ounces at a time until I had the right proportion. Since the coffee extract is very strong, the flavors in that batch are strong but they should marry up with time.
I have 2 more caramels and 1 more coffee kit so I'm really tempted to make a coffee/caramel blend. I'll try mixing them in a decanter first and see how good that is.