It's definitely working -- in fact, the FireTV won't even display the "4K Ultra HD" categories unless it's connected correctly to a compatible TV[0]. I notice the difference if I actively try to, I just don't think it's as mind blowing as everyone is claiming it to be. It's certainly not something I couldn't live without.

[0] https://www.amazon.com/gp/help/customer/display.html?nodeId=...