Recently, several patches landed that improved the status of the project. You should expect the device to be booting, be able to set the date/time. If you have a SIM card, it should be able to unlock, register on network, automatically configure APNs and connect to data network. Wifi should also work.
Pinned sites have been fixed and the homescreen is able to pre-populate a couple of apps: browser, settings: thanks @benfrancis and @Cwiiis and @albertopq. So you should be able to browse the web! Playing videos on youtube is something that is expected to work (although I had no time yet to verify).
The white layer on homescreen and keyboard should also be something fixed, thanks to @kanru!
On the low-level side, B2G specific APIs are expected to be exposed ONLY to chrome:// pages. If you are able to make use of any of those APIs from content (http/https pages), then file a bug and ping us. Removing is not something very hard and could be a first patch :).
An up-to-date Z3c blobfree build can be downloaded out of TaskCluster from pine repo: https://tools.taskcluster.net/task-inspector/#bHwYD8w_RqCD6Pd3d9OQQw/0