Just saw it pop up, too. So far, nothing too controversial I can see. 3 successful installs at this point, 2 more to go. Let us know when the update page catches up, please!
--Ed--

Here's what the Update page says:

March 8, 2016 — KB3140768 (OS Build 10586.164)

This update includes quality improvements and security fixes. No new operating system features are being introduced in this update. Key changes include:

• Improved support for Bluetooth, wearables, and apps accessing contacts.
  
• Improved reliability in app installation and Narrator.
  
• Improved performance for hibernation, content entry in apps, and downloading and installing updates.
  
• Fixed issue that didn't allow login to an Xbox from a PC running Windows 10.
  
• Fixed security issue created when attempting to play corrupted content.
  
• Fixed security issue that could allow remote code execution while viewing a PDF in Microsoft Edge.
  
• Fixed additional issues with .NET Framework, Internet Explorer 11, and networking.
  
• Fixed additional security issues with Microsoft Edge, Internet Explorer 11, USB storage driver, kernel mode drivers, .NET Framework, graphic fonts, OLE, secondary logon, PDF library, and Adobe Flash Player.

Mine started downloading but has now been stuck at 48% for the last ten minutes.

DiBBz said:

was playing a game and noticed i was getting horrendous performance and seen that windows was downloading updates.. however it seems that im now stuck at 48% and it wont move.. ive been sitting here watching it for half and hour and its simply wont move!any help ?

Same as me then. Just leave it, I walked away from my PC to start making a meal, checked again about 15 minutes later and had started installing.
All completed successfully.