Once Amazon Prime comes to the Apple TV, I? would say that it is as good as it needs to be.
Absent Amazon Prime, it is bit of a weak sister as it stands today. Going forward, it will continue to suffer from the absence of YouTube 4K support by decree.
The Wired UK review indicates that the Apple TV 4K is the only box on the market that automatically upconverts everything (and apparently not very well in some cases) versus playing it in native mode. Apparently if you want to view something in native mode, you have to go through the menus to change the output to whatever mode is appropriate.
The 5.1 sound limitation is probably okay for most who would otherwise be watching iTunes on a tablet or phone.
Of course this doesn't consider what Apple could theoretically do (if dragged, kicking and screaming) or what they have hinted that they might do (perhaps adding an auto-switch output mode).
For that kind of money, one should expect a box that is substantially unconditional in its capability. Promises of future apps and games are just words when you're going up against some very established players.