The Nexus 5X is going to get a very quick software update. In fact, as soon as folks start getting the phone in their hands, they may see the OTA update version MDB08I arrive on their device.
This "new" version actually brings the phone up to the same release as can be found in the factory images. This suggests the update is just a few bug fixes and security patches. The update is around 42MB, so be prepared to grab it over the "why-fies."
Here's our dedicated Google Nexus 5X section for further discussion: Nexus 5X (2015) | Android Forum at DroidForums.net