Microsoft has released Windows 10 Mobile Build 15208 to Windows Insiders in the Fast Ring. Windows 10 Mobile Build 15208 is from feature2 branch and it comes with improvements for the operating system. Microsoft has also released a new build for Windows 10 PC users in the Fast Ring to Windows Insiders.
Microsoft has added no new features to Windows 10 Mobile with Build 15208. However, Microsoft has fixed some bugs with this release. Microsoft has finally fixed an issue where you were unable to open connect UX page or add any new Bluetooth or devices on the Bluetooth Settings page. Your phones will now shutdown quickly as the company has fixed an issue which was causing the phone to stick in deadlock for more than 20 seconds.
As it is a preview build, it does come with some known issues. Some Windows Phone may lose the text message backup. Furthermore, your phone could misbehave and shutdown without any warning.