Update 1: 2017/10/13 3:59am PDT
Motion Photos work on v5.0 on first-gen Pixels only
After the updated Google Launcher from the Pixel 2, we have the APK file for the updated Google Camera app that will come on the new Pixel as well. You can install this one on your current Nexus, Pixel, or other OEM-branded phone and give it a try to get a taste of what the Camera app will be like on the Pixel 2.
[Update: Motion Photos with v5.0] Hands-on with the updated Google Camera 5.1 app from the Pixel 2 [APK Download] was written by the awesome team at Android Police.