AOSP on db820c is in a pretty decent state as of today. HDMI display and Audio out works (Freedreno), WiFi (STA, Hotspot) works, BT (HID, Audio, Tethering) works, USB gadget (ADB, MTP/PTP, Tethering) works, USB Host (HID, Ethernet) works, On-board Ethernet port works.
Project is still in development stage and here are the open bugs:
db410c/db820c: webview crash https://bugs.96boards.org/show_bug.cgi?id=749
db820c: qcomlt-4.14: random linux boot failures https://bugs.96boards.org/show_bug.cgi?id=738
And there are few known UHS Sdcard initialisation issues we are tracking at the moment.
You can download daily builds to smoke test it on db820c http://snapshots.linaro.org/96boards/dragonboard820c/linaro/aosp-master/. Last daily build being tested was build #102
AOSP build instructions for db820c are hosted at https://www.96boards.org/documentation/consumer/dragonboard820c/guides/aosp/