Ok, I have taken the plunge and installed Windows 10 on my M8, it definitely feels more like a beta than it does for my tablet or laptop. There are some issues with how things look and there are some really slow spots in it as well, seems like mainly when trying to go back to the Start screen and when resuming from an app. I have also noticed that apps and windows seem to stay open after you click the back button, so after a few minutes of using it you could have loads of windows open that you would need to press and hold the back button to close them. The mail app is getting very close to what is on the tablet and laptop, swipe to delete etc. It also seems like the settings area is much better looking than it used to be in WP8.1, although it seemed to look more like a phone settings before. I do not think it is for the feint of heart to install this on your main phone, but it is nice to use it for the most part. There is a setting for having more buttons visible, but I could not get it to take setting it back to do not show more tiles. Another thing about this version, what are they going to call it? It seemed like it would be Windows Phone 10 since that was just the continuation of names through WP7 and WP8, then it seemed like it would be Windows 10 since it was all the same code base, now it seems like it may be Windows 10 Mobile, resurrecting the Windows Mobile from the Windows Mobile 6 days.
One thing I did notice was after the installation happened I saw an update for WP8.1 in my phone updates and could not get it to go away, until I went back in the Windows Insider app and set the setting back to Fast Track, which I had set before so I could get the update in the first place. Once I did that the WP8.1 update disappeared and I have been seeing Windows 10 updates showing up. Speaking of updates, it seems one thing they have not finished yet is a way to force a restart if you happen to miss the window telling you about the restart, I have seen it a couple of times and sometimes it shows up in the update area as a button, but if something goes wrong there is no way to force the restart, and I do not think holding the power button is a legit way to restart.