I bought a 64GB 4K Apple TV recently and it's magic.
It replaces a combination of Fire and NowTV sticks. Both of which are old and were cheap at the time. Their UIs are slow, remtoes feels cheap, and clutter up the place.
My immedate reaction is that tvOS is buttery smooth. Zero complaints. Moving between apps (and having it remember positions) is perfect. Starting streams is super fast, especially NowTV!
Video looks sharper. We have a 1080p TV but I suspect it's handling the video decoding better than our old devices.
Colour correction was a fun experiment. It made things slightly warmer but not really a noticable change.
The remote is a huge improvement over the previous monstrosity. It has a lovely weight and balance. The clickwheel type control doesn't get in the way. Plus it's rechargeable so no need to keep AAA batteries!
The most joful experience is the password manager integraton. Logging into apps on tvOS prompts me on my phone to fill the password field with my password manager. Magic.
The one downside has been the iPlayer app. It does not currently provide subtitles which is frankly unacceptable. There's a statement on their site saying:
There are technical challenges associated with delivering subtitles to Apple TV which will require a significantly different solution to that which we use on all other platforms. We are working towards it but don't currently have timelines associated with this support.
We sent a complaint and got a similarly hand waving response. Sort it out!
Finally, it has reduced the clutter. Two remotes into one. One HDMI port. One powersocket.
At more than twice the price of the old sticks, do I think it's worth it? Absolutely.