While the app is not officially released on the Google Play Store, it seems to work on most, if not all devices, and is the unaltered Google app from the Google Play edition phones (and likely soon Nexus devices). While it isn’t on the Play Store, you just download and install it like any apk, no need to have a rooted phone. The apk also includes the stock Android gallery app.
The new app features a redesigned menu, stepping away slightly from the circle that appeared last year with Android 4.2 Jelly Bean. The new app is almost the same in terms of functionality, but has a semi-circle, with an addition semi-circle popping up for ‘more options’.
The best thing (in my opinion anyway) is that it comes with the photosphere 360 degree panorama mode. Something that is still officially only available on devices purchased directly from Google.