Ok, so I got one. Looks pretty nice, stays pretty quiet. I have an incredibly quiet apartment, so I can hear it, but it's not loud at all. Any volume from my stereo covers it easily. Xbox One I could barely hear at all, from where I was sitting.
Had all of my stuff on an external so setup was easy. I'm using a 1080p screen right now, so I'm getting the supersampled result.
Gears 4 looked good in the level I was playing, but I preferred the 60fps performance mode to the supersampled look. I know the 4k mode has other visual enhancements, but I can't remember what. The level I was playing may not have been the best example. Still think I'd pick 60fps on a 1080p screen.
Quantum Break looks massively improved. The image quality is just way more stable. Still a unique looking game. Might play around with some specific levels.
Tomb Raider seems to have some kind of bug. I was playing the DLC a while back, and didn't finish it. Tried to continue my game and it said it couldn't load the save because the dlc isn't installed ... but it is installed. Will check it out later after deleting and reloading the dlc.
Titanfall 2 looks very nice supersampled. Only played multiplayer, but you can see into the distance much more clearly. Textures stand out as kind of low res in a lot of places now that the image quality is so sharp.
FIFA 18 looks basically the same.
Killer Instinct looks clean, but I don't really play that game.
Battlefield 1 is not enhanced yet, but it actually has very nice results. For one, it seems to stay at 1080p and not dynamically scale down. Also, the framerate is much more stable in multiplayer. I didn't see any noticeable drops, but there could have been some minor ones. It's a very noteworthy improvement even without enhancements.
I think that's all I've tried so far. Overall, I like it, but unless you're really into image quality, there probably isn't much reason to get it yet, unless you have a really nice 4k HDR tv. As more games get patched, and new games come out that take advantage of it, it'll be an easy win if you can afford it, even on a 1080p tv. Battlefront 2 looks pretty fantastic, and would look massively better than the Xbox One version on a 1080p screen.