I got my Sony UBP-X800 today. I can confirm that it is actually a 4K UHD Blu-ray player and plays back 4K discs.
I don't know what the official release date actually was but it seems that Best Buy is selling it early because the network features and apps are mostly not yet enabled. Netflix works, all other apps cannot launch. Playstation Video just gives an error saying the service is not available. There is no promised release date yet on firmware which enables the apps.
It plays back 4K discs just fine and is the fastest player I've tried between Bone S, Samsung, and Philips in terms of loading discs and navigating menus. I have not tried the Panasonic and Oppo players. Of particular interest is the fact that this is the first player which features an adjustable HDR to SDR conversion algorithm, when the movie is playing you can twiddle a slider up and down which changes the luminance and saturation values of the conversion.
Obviously if you have an HDR TV this is meaningless to you. Those of us who have SDR TVs will find it interesting. Playing with the slider did make the image brighter or darker, though without an HDR TV to compare side-by-side it's not possible for me to see which SDR setting is most "accurate" relative to an HDR image.
The Blu-ray disc playback is the fastest I have ever seen. It's The Flash in terms of perfomance. It's the Blu-ray player version of
"I'm running so fast that I'm my own hype man!" You put a disc in and almost instantly it has loaded. Reportedly this player has a 10-core Mediatek SoC in it, the same one in the Oppo, which is probably why this thing runs like it's literally on fire. BD playback is otherwise what you would expect from any modern BD player, after 10 years I think any player you buy today can play back BD just fine.
I really wonder how much better the Panasonic and Oppo players can be for more than twice the price. In terms of 4K UHD BD playback, it curbstomps the Samsung UBD-K8500 into the ground, I had issues with the way the Samsung player handles HDR to SDR conversion among other things. The Philips BDP7501 that I own only cost me $96 on Black Friday so I can't help but give it a pass for being by far the slowest player by a very wide margin. I couldn't stand the sluggish bloated UI of the Bone S, plus at the time it didn't support bitstreaming of audio so it had to go back after a week. For $300, this thing is really good so far. Although I kind of hope they enable the apps soon, hopefully that happens whenever the actual street date is comes around.