I'm quite disappointed to see only an f2.0 aperture lens for the main/rear camera. I was expecting an improvement in that aspect. Xiaomi did use a larger 1/2.9" (compared to 1/3.06") sensor with 1.25-micron pixels (compared to 1-micron) to help improve the low-light performance and image quality of the rear-facing camera. There is also the 4-axis optical image stabilization that also will help to make sure that images look sharper and videos are less shaky compared to the original Mi Mix that lacked this feature.
So no doubt that there are improvements that should allow the Mi MIx2 to take much better pictures in low-light and promote better videos. That being said, the f/2.0 does stand in the way to make it a stellar performer. I've seen phones like the Google Pixel that were able to get out of it with a relatively slow lens and no OIS, but that performance was achievable due to a relatively large 1/2.3" sensor size and large 1.55-micron sensor pixel size. The Xiaomi Mi Mix 2 has smaller 1.25-micron pixels and smaller sensor.
According to gizmochina.com website, the Mi MIx 2 uses the Sony IMX386 sensor, the same sensor of the Mi 6. It's a very popular sensor that exists in dozen of other phones from Huawei, Meizu and Xiaomi.
The question is whether the 4-axis OIS will be able to compensate for the slower aperture. I wonder how many effective f-stops the OIS of the Mi Mix 2 is capable of. I haven't seen many websites trying to measure this, let alone that it's not even mentioned on the official mi.com website. Once the Mi MIx 2 is compared to the Galaxy S8, iPhone 7 and LG G6 cameras, we are probably going to get a clearer idea how good that camera is and how it is compared against the other premium smartphone cameras out there.