My experience with set-top boxes like Apple TV are: Roku, Apple TV and Samsung SmartTV. I've seen, read about and demoed some of the others, but I have used the above regularly in my home.
Apple TV is my favorite remote, voice control, and interface.
When I bought the 4th gen, I was excited about the prospect of gaming on it. While there has been some cool games added, there really isn't anything there that makes any kind of statement about the device as a gaming machine. There's no "killer app" for Apple TV. You get most of the popular channels/services and some great mobile/indie games. So it does the job and it does it fairly well comparatively.
I don't own a 4K TV yet and don't expect to for a while, but I do want 4K and HDR to be supported by Apple TVs going forward so that when I do upgrade, my new Apple TV will support it.
The speed of the 4th gen is still pretty great with its A8 variant. Of course any update to the SoC is going to introduce good opportunities and improve the responsiveness of the interface.
The remote does a surprising amount for how few inputs it has. It may not be as good as a gaming controller as Apple wanted it to be, but for navigating apps it gets the job done well. Many people don't realize there are multiple buttons in general touch area of the remote which adds some neat features when apps use them.
I would like to see tvOS add the ability to use multiple apps at the same time. Or, at the very least, use PIP, like people can do with video on iPad (where video can play while user does other things). I would like to be able to watch something on YouTube while I browse for either a new YouTube video or browse Netflix for a TV show or movie to watch. It sucks not being able to look for the next thing to watch until you're done with the current video.
AirPlay is wonderful, but it has glitches. Any improvements to AirPlay are welcome.
While many of the popular services are on Apple TV, some of the apps don't support the same TV Providers as their iPhone/iPad counterparts. For example: on iPhone/iPad the MTV app includes Comcast/Xfinity. However, the TV version does not. This is frustrating and a reason I often use AirPlay - by launching the iPhone/iPad version and casting to the Apple TV.
The new TV app is terrible and useless for my use case. Thankfully you can set it up so that the home button goes straight to the home screen instead of the TV app. The universal login doesn't support many providers and even if they did there would still be restrictions like with the MTv app for whatever reasons.
Glad there's new hardware coming, it more looking forward to the software updates; hopefully most, if not all, of the software updates are compatible on the 4th and 5th gen Apple TVs.