Regarding the Fast ring update, here's what Microsoft had to say about build 15055:
Other changes, improvements, and fixes for Mobile
- We fixed an issue from recent flights where the volume control might stop working when listening to music in Groove after an endpoint change (for example plugging in headphones).
- We’ve improved speech reliability by fixing a bug where Cortana was unexpectedly failing with a connection error.
- We fixed an issue where multi-turn Cortana speech interactions weren’t working (for example, when creating a reminder in multiple parts, as opposed to doing it in a single sentence).
Known issues for Mobile
- We have discovered a bug that causes background tasks to not run in the background like they should such as OneDrive’s camera roll sync feature. Please double check your photos have synced to OneDrive on the latest builds. We’ve got a fix for this issue coming in an upcoming build where background tasks will run again and your camera roll will sync to OneDrive as expected.
- If your device experiences an unexpected reboot, in some cases you may lose your call, text, and email history may be lost. We’re actively investigating this issue. If you believe you’ve hit this, please up-vote this feedback item In Feedback Hub.
- There is a bug in this build that can cause a number of inbox apps to fail to launch (such as Store). There is no workaround to reset the Store unfortunately. This issue also prevents you from taking any app updates from the Store if the Store works for you. A workaround for this is to uninstall the app and reinstall the app from the Store. This issue will be fixed in the next build. Speech packs may be unable to download on this build.
- Some users are reporting that pages are constantly reloading or refreshing, especially while they are in the middle of scrolling them in Microsoft Edge. We’re investigating.
How many Fast ring builds will we get before Redstone 2 will finally 'ship' to production phones? My guess is another two, taking us to the last week in March, following which we'll have a short pause for final sign-offs. And then the official release and then some extra 'Release Preview' builds to tidy final bugs up through April/May.
Which means that at some point soon after the Creators Update hitting, many Fast ring Insiders here might like to consider switching to that (Release Preview) ring before the end of May, otherwise you'll start to get shunted off into Redstone 3 early builds, and those will be pretty unstable for a while. Just something to think about!
Meanwhile the Slow ring was pushed build 15051, already covered here. Sounds like there's still some work to do overall - I'm running devices on all these rings and will report back in due course in a full editorial.
In the meantime, anyone sitting back on a Windows Phone 8.1 smartphone (think Lumia 640, 735, 830, 930 etc.) might like to read my 'How to upgrade smoothly from Windows Phone 8.1 to Windows 10 Mobile' feature, in preparation for the official arrival of the Creators Update (a.k.a. Redstone 2) - clever money has this being pushed to 'production status' devices in late April. So no hurry, but it's definitely a good time to make the move and get used to the revamped interface.