I just received my Moto X4 from Google the other day. This is replacing my Nexus 6p, which I was concerned about.
After spending a few days with the device, I am thinking I made the right choice. I had concerns for camera quality, and while the 6P takes better shots outright, the X4 camera application is far better than that found on the Stock Android camera on the 6P. So if you use the manual settings, you can take fine pictures with the X4. Otherwise, the colors are cooler, the shadows are a bit crushed, and a little less detail.
Otherwise, the phone seems to be an improvement in nearly every other way. Single thread performance isn't as strong, but multithread is stronger. I believe that multithread performance is likely more important here anyway. I haven't noticed any sense of sluggishness in any of the individual apps I use. It seems quite snappy.
The display is bright, and I don't even notice the lower resolution. The display colors are cooler than that of the 6p, and it looks less dingy in comparison. I am pleased with the display.
It only has 32gigs of internal storage, and the SD card is not adoptable. I have yet to determine that this will be a problem. I suspect that it won't be.
I am much more confident in the build quality. The phone is slightly thicker, made of more glass, while the other proportions are smaller. It feels better in the pocket and hand, and I worry less about bending or breaking it. I backpack a lot and so I worry less about water damage now too.
The batter is leaps and bounds better. I used the device yesterday well into the night with what I regard as heavy use. The 6p would have crapped out around 3 in the afternoon and at around 20% battery.
While the hardware and software seem to be more modest, it seems that the phone is much more refined than the old 6p, and so worth much more than the sum of it's parts.
My biggest concerns regarding the move to the X4 was regarding call quality, durability, the camera, and battery life. I was on my 3rd 6p. Call quality was generally terrible, from day one. Battery life was never exactly great, and degraded to terrible over time. The phone generally felt fragile and just large enough to be cumbersome.
In short, I think I am very pleased with the Moto X4.