I have an axe to grind with Google TV. Got the device today. Coming from Roku and miss the simplicity of the home screen. Tried app-only mode and don't understand why it shows all apps in a single scrolling row -- what about filling any of the other real estate on my screen? Also, once app-only mode is enabled, one cannot use their personal Google Photos in ambient mode anymore. For an inexplicable reason, that option goes away when I enable app-only mode.
On the bright side, my Logitech Harmony Hub paired with it via BT without issues.
While I'm surprisingly OK with the rows of suggestions from (some) of my streaming services, I find the default UI to be limited as well. You can accomplish most (but not everything) of the things possible with the classic UI, but it's more convoluted. Android TV on my Sony TV is straightforward, with an easy way to set up favorite apps and making them obvious while the rest are in the app drawer. It provides straightforward access the app store, and more fine tuned settings. I'm getting used to the setup, but I'm not happy the old UI is going away.