The Sony Xperia XZ1 rear-facing camera can shoot pictures at ISO 12800. The Xperia Z3 can do that as well. This means that users can shoot at very dark conditions, although it does have a negative effects on the image quality. For movies, users can shoot up to ISO 4000 for low-light videos. It's great to see such an advanced Sony camera sensor that is capable of that sensitivity. I mean, I know many DSLR cameras that don't reach that ISO sensitivity.
Keep in mind that although the broad ISO range is a nice feature, it doesn't mean that it will be capable in giving you great low light shots every time. Of course modern sensors, especially those Exmor RS sensors from Sony, are capable of delivering very impressive results in low light. Furthermore, the Xperia XZ1 comes with a large 1/2.3" sensor (relative to other mobile phone cameras), but I think that its high 19-megapixel resolution and 1.22-micron size pixel would limit its capabilities. It also uses only an f/2.0 aperture lens, which doesn't help to improve its low light capabilities either. It was nice to see a faster lens in that regard.
Here is a video showcasing the ISO 12800 feature in the Xperia XZ3.
Sony loves making high resolution sensors and I actually wish it had produced them with significantly less resolution, so we all can enjoy superb low light pictures without needing to climb up that much on the ISO scale.
The idea of offering ISO 12800 option is that users could should in extremely dark lighting conditions and still be able to get usable shots. Of course resized low-res images of pictures shot at higher ISO will look better compared to full scale high-resolution ones. Even some DSLR cameras that sports APS-C or 35mm Full-frame sensors struggle at that ISO speed, so a 1/2.3" will definitely will.
So overall it's a nice features to have and it can be useful at times. For some scenes it might be a matter of getting or not getting an image.It also allows you to take pictures without flash. I'm sure that the XZ1 image processor unit has some nice tricks behind its sleeves to make the image look better and some heavy processing is done behind the scene to reduce the amount of noise as much as possible. I recommend using ISO 12800 only if there is no other option to get a particular shot in any other way, or if you want to shoot at higher shutter speed, so you need to boost the ISO to compensate for the lack of light.