Mixed emotions....
Received my long awaited new MM on launch day. So far, it's okay. Not great, not sucky, just okay. I am very frustrated with the video. Sometimes it boots, goes to set the resolution, and then a black screen. Sometimes it registers my 4K display as 4K. Once I even had it register the 4K display as a 5K. I wish! Never had that problem with my old MM, once the resolution was set, it was consistently good.
Troubleshooting so far has been swapping cables & HDMI ports on the display. Every cable and HDMI port runs 4K just fine when it is coming from 4K Blu-ray player. However, only one cable works with the MM, and even then it is spotty on boot up. Got the 'meh from Apple support... "Boot in to safe mode once, that resets everything, you should be fine now."
The upside? When it is working, it does great.