The Xperia XZ we have here is running Android 6.0.1 Marshmallow out of the box. Nougat is all the rage now, but we doubt it Sony will have enough time until October, when the phone is scheduled for release, to make the jump to Android 7.0. A safer bet would be, that it will launch on Marshmallow, with Nougat to follow a few months later.
Sony is keeping the stock Android look, except for a few gentle touches here and there. The proprietary Stamina battery saving modes and the home-baked multimedia apps go without saying.
The fingerprint reader is a recent addition to the Xperia line and Sony firmly believes the best place for it is on the side, integrated into the Power button. We can't deny the convenience, as waking the screen and unlocking the phone are tightly related.
We had an excellent user experience with the fingerprint reader on the Xperia XZ. It's fast and accurate. There's an animation that slides the lockscreen away from your thumb (as if you pushed it out of the way), which helps hide the small delay, so it feels seamless.
Smart Lock gives you conditional security - trusted nearby devices, locations, faces, or voices can allow you to skip the security unlock protocol.
The homescreen appears unchanged from the rest of the X series. This includes the swipe down gesture, which shows a screen of the apps you use most along with recommendations for new apps to install. The search field is highlighted so you can start typing the app's name immediately.
The traditional app drawer is present and it features an above average amount of vendor apps. Sony takes great pride in the A/V prowess of their devices, so they have pre-installed a nice set of their own multimedia apps.
Themes are available (both free and paid) that can customize the look and sound of the Xperia XZ UI.
The notification area is plain Android. You can re-arrange the quick toggle tiles and adjust the screen brightness. Note that just like in vanilla Android; there's no toggle for Auto brightness (you need to go into the settings for that). That's the single thing we don't mind getting changed by manufacturers and still, many manufacturers like to keep this aspect stock.
The app switcher is similarly a vanilla Android affair with the 3D rolodex look. The small apps are gone, however, and there is no longer floating-app multitasking. There isn't screen pinning either like we had on the Xperia X Performance.
The Smart cleaner feature will periodically empty the cache of apps you haven't used in awhile. You can switch this off or just manually tell it not to bother for certain apps.
One thing Android has been missing for years is a proper backup solution and Sony gives you one. It can backup applications, contacts, messages, phone settings. The backup info itself can be stored in the cloud under your Sony online account, or locally on the microSD card or an external USB device.
Backups can be scheduled, including conditions like "Connected to Wi-Fi" and "Charging device", depending on your preferences.