Build 10166 works on Lumia 520.
The last build bricked my 520, had to use WRT to recover.
This build is working, thanks for tips from other posters.
I upgrade from WP8.1 then did a hard reset and did not restore from backup.
Make sure your phone is fully charged. My phone was on the charger while upgrading, but the battery still ran down, was not charging during upgrade.
Upgrade started adding Apps next, then the digital keypad came up and it appeared hung. So I did a soft reset by pulling out the battery.
Seems to be ok now. I did have a problem when I tried to remove some home screen tiles, the phone rebooted and the tiles were still there. Resizing home screen tiles does not work, they go back to original size. WP did not find my phone numbers saved on my SIM card, only found contacts saved in Outlook.com
Apps are very slow to load, but worked smoothly once started. I updated Here Drive+ and downloaded maps. It turned on GPS and used it this morning on the way to work, no problems.
One big complaint, too many screen touches. To change ringtone: settings>system>personalization>sounds>ringtone.
This is my daily phone, if it’s ok today, I may keep running it.