First up, unlike the round Echo family, the FireTV Cube is, well, square. It's quite sleek. A glossy, piano black cube that sits on your TV shelf or console.
It comes bundled with Amazon's ethernet adapter (approx $15) which is a good thing and allows a wired connection to your internet router or modem for a more stable connection.
Connections at the back are for power (oddly a separate plug instead of over-USB), a micro USB plug for the ethernet adapter (unsure if power can be supplied over this), HDMI, and a port for the remote IR extender.
Setup is quite easy, place the included batteries in the remote, plug in all the cables and power up the cube! Everything else is voice and screen prompted to guide you through. If all your devices are connected via HDMI (TV, AVR, Bluray player, etc) then you may not even need the IR extended. The FireTV Cube will be able to send commands over HMDI to control all your other devices.
As you go through the setup process, the cube will run some tests to determine compatibility with your devices. The FireTV Cube has most of the major brands already built in (LG, Samsung, Sony, Onkyo, Denon, etc) so selecting them from a drop down list is easy. For those that are not yet on the list, Amazon promises constant updates and generic control via the IR emitter. Once done, the Amazon FireTV will have a decent amount of control over your home entertainment system.
So, how well does the cube control all your devices? Quite well, if you manage expectations. What does that mean? It won't replace all your remotes all the time, it will replace all of your remotes most of the time for the most common functions. Power, volume, input switching, and channel selection can all be done by voice. When watching videos through Amazon Video, voice commands can do a bit more. For example, if you searched for Dare Devil, the cube will display several results in a numbered list and just say "Alexa, play 1 or Alexa play 2" depending on which result was the one you were searching for. It's not very smooth, but its a good system for selecting from similar results. Or if you know of a show on Amazon Video, just say "Alexa, watch [name of show]". The Cube should pull it up and start playing right away.
You can also set the volume increments when you say "Alexa louder" or "Alexa softer" to control the volume. I prefer larger jumps otherwise you just keep repeating the command over and over.
This may or may not replace any universal remotes like the Harmony. But it does offer a much simpler way to interface with a complex theater setup.
Other than entertainment system control, the FireTV Cube also works with your smart home. If you have a Ring doorbell, you can ask "Alexa, show me the front door" and it'll connect with your video doorbell and display the video feed. A nice novelty to have. This feature also works with the other FireTV units, but being able to do it without the voice remote (like with the Amazon Echo Show)
Now, the not so great part. Oddly, Amazon calling is not available on the FireTV Cube. This is probably the weirdest omission on Amazon's part. Hopefully this will be corrected in future updates.