Software
The Xperia Z5 runs Android 5.1.1 out of the box, and immediately this is a disappointing sight to see: Android 6.0 has just launched on the new Nexus devices, and considering Sony's pretty terrible history with timely handset updates, I doubt we'll be seeing a major update for this phone any time soon. Really this device should have launched with Android 6.0 on-board, but clearly Sony wasn't ready for this.
The good news is that Sony's Android skin has matured significantly since I last looked at it on the Xperia Z3. The general design of the skin is much closer to stock Android than anything the company has produced before, while still retaining some signature Sony features that fans of their phones have come to enjoy over many years.
Some of the most awful aspects of Sony's Android skin on the Xperia Z3 have been completely removed on the Xperia Z5. The notification pane, for example, is simply the stock Android 5.1 version, rather than the plain black atrocity that was used in the past. This is both a significant visual and user experience improvement, and I applaud Sony for realizing when Google's version of a feature is clearly superior to anything they've been able to develop.
Sony's homescreen, lockscreen and widget setups aren't anything amazing, and all have received minor improvements over previous versions. I appreciate the thin and stylish font Sony uses throughout apps and the homescreen, as it makes the display look sharper than its 1080p resolution would suggest. The large digital clock on the lockscreen is also exactly what I want to see when I power on the display.
The stock applications used throughout the Xperia Z5 have been completely overhauled to use Google's Material Design, rather than the dated and lacklustre style used previously. Apps such as Contacts, Calendar and Messaging are all easy to use, clean and attractive from a visual standpoint, and cohesive with the rest of Android's design. There aren't a whole ton of interesting features in these basic apps, but the design is exactly what I want to see from an Android skin.
Media consumption has always been a big focus of Sony's Android devices, so the Xperia Z5 is loaded with media-related apps. The Album (gallery) app contains a lot of features, including photo aggregation by location and faces (by facial recognition), and the ability to browse photos on your local network. The Music app is a similarly great experience, although it's not as useful as many people have switched to streaming music services.
The Xperia Z5 can also connect to a PlayStation 4 through Remote Play so that you can play games on the phone's display. Personally I don't have a PS4 to test this, but it's a compelling feature for gamers that might not want to play games on a television.
Unfortunately, the Xperia Z5 includes a ton of bloatware applications that should never be included on the handset in the first place. Apps like AVG Protection, OfficeSuite, Spotify and AAStocks are from third-parties and should never be installed in the first place, while apps like Privilege Plus, What's New, News from Socialife, and Lounge are largely useless for the average smartphone user. On top of that there are a bunch of very average first-party tools, like Lifelog (fitness tracking), Sketch (drawing and doodling) and TrackID (music recognition, which Google Now already does).
Luckily most of these bloat apps can be uninstalled, and I'd highly recommend you do so to unclog the app drawer.
There are some interesting settings that Sony has added to Android for the Xperia Z5, such as basic theming options and the ability to show or disable system icons. As for more useful functionality, there are quite a few audio tools that can customize how music sounds through headphones, including a dynamic normalizer and a system-wide graphic equalizer. There's also Sony-made backup and restore functionality.
Finally, the keyboard on the Xperia Z5 is decent enough. It has swipe support, as well as fairly a reasonable prediction engine, and an easy-to-access emoji section. The typing experience is okay, though sometimes it can be a little slow, so there is a case to be made for switching to a proven third-party keyboard like SwiftKey.