Microsoft has finally rolled out the Windows 10 Mobile upgrade to select Windows Phone 8.1 devices.
Here’s the current list of phones and devices that can upgrade to Windows 10 Mobile:
Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, and MCJ Madosma Q501.

Window 10 Mobile brings new features to new and existing supported devices like Windows Hello, refreshed UI, Continuum, Edge browser, Cortana, more functional Action Center, and support for newer apps.
To check if the upgrade is available for your phone and install the upgrade, go to the Store, install the Upgrade Advisor app, run the app to see if the update is available, and then select Enable Windows 10 upgrade.
source: Microsoft

I updated my Lumia 636 yesterday. It seems to come in two parts, first an update to 10586.107, and then when you check for updates again, another (just as long) update to 10586.164. Seems to work fine for the most part. It took about 5-6 hours total, though.
The new “Groove” music player is really horrible, though. It doesn’t read my mp3 tags or album art.