The Xperia Z5 was Sony's 2015 flagship phone, however it will be replaced by the Sony Xperia X Performance.
Pro Dedicated camera button
The dedicated 2-stage camera button makes launching the camera app and taking pictures just a bit easier. You can press the button half way down to focus, and then pushing the rest of the way will take the picture instantly. To quick launch the camera, push the button (works even while the screen is off).
With an IP68 certification, you won't need to worry about getting the Z5 wet. It is at the very top of the IP rating scale, but still susceptible to damage from pressurized water (either spraying it or taking it deep under water).
Con Stagnant design
Sony hasn't updated their design in any significant way since the Xperia Z first launched in January 2013.
Con Runs warm
While it doesn't get uncomfortably hot, the Z5 still pumps out more heat than most other phones. It isn't the worst out there, but it certainly isn't one of the more thermally efficient phones out there.