This is the first build I got for my Lumia 1520.
1) Installed on Windows 8.1
2) Most of the apps were not working, couldn't open settings
3) Did hard reset by pressing Vol Down and Power, then once vibrates hold Vol Down, then when "!" is displayed pressing sequence Vol Up - Vold Down - Power - Vol Down
4) Did clean install, no backup.
5) Couldn't install Russian keyboard - had to re-enroll to Insider Preview. Worked.
Issues:
Groove doesn't have an option NOT to include music from OneDrive. Very annoying.
Groove sometimes doesn't switch to next song, especially on shuffle. It shows PAUSE button, meaning it thinks its playing when it is not. Annoying.
No "Hey Cortana" - it worked bad on W8.1 so I am not missing it.
No motion data - health app without Band (and there are no bands in Estonia) is useless.
Unlocking the phone only moves the text up - very weird looking effect. It got to be a bug. Really weird.
No way to set Weather app for the LockScreen - that is the most annoying part. JUST horrible.
No heating and battery draining so far.
Not a daily driver yet... But since I am lazy I will leave it. We will see.