Following the release of a few new Windows 10 builds for the desktop, Microsoft has released build 10166 for mobile. As with all of the recent builds, Microsoft's biggest focus right now is refining what's there, to make sure that at official launch, the OS is as stable as possible. 10166 is no exception.
Requiring an opt-in to the "Fast" ring, 10166 fixes numerous known bugs found in build 10149, including an issue where the lock screen would appear frozen after an upgrade. Likewise, a strange pin entry bug has been fixed, one that allowed you to keep typing in your pin even after the correct one was entered. Some aesthetic fixes have been applied as well, notably to the accuracy of the colors that appear in titlebars.
Also worth noting is that the reliability of app downloads from the store has been vastly improved, and tying into that, the store is no longer denoted as a beta.
Since this build just came out, and is part of the fast ring, you may wish to wait before upgrading. If you're a bleeding-edge kind of user, you should be able to update right now.