Update: Having owned 8 of these purifiers for more than a year, I can report back that the scripts side of the Xiaomi Mi Home app, the automation that is supposed to control the air purifiers, no longer works. I'm a programmer, so if anyone can make scripts work, I can. They worked flawlessless for a couple of months, then they suddenly all stopped working. The scripts run on the Xiaomi server (not locally), and the server stopped running the automation.
After many attempts of contacting Xiaomi for support through many different channels, I got zero responses. I felt it necessary to update my review here with a more complete version of my experience.
Original Review: I've owned air purifiers for many years, since we have a neighbour with a very smoky wood heater. The previous brand I used no longer sells purifiers, so that lead me to Xiaomi. As a result, I'm in a good position to compare the brands.
Firstly, this unit is quite big, but bigger is better, in the sense that it can push through a lot of air. But you'll need to find room for it. Ideally, that should be on a table off the floor. I find the units quite attractive, like little statue works of art.
The standout feature of these purifiers is the 2.5 micrometer air polution meter: it has a screen which constantly tells you the particle count. It uses a separate compartment with a laser that counts the particles. So very high tech.
Based on the reading, it adjusts the speed of the fan, ramping it up as needed. This is an amazing feature, and works really well. In particular, when the polution is low, the units run almost silent, and the fan noise only ramps up when absolutely necessary. So 10/10 for this approach. The only caveat, is that the Australian Standard for average air polution across 24 hours is 25. These units won't ramp up at that reading. Disappointing. Instead, it uses the Chinese air quality limit of 75. Being a Chinese product, that makes sense. But you're unable to adjust that. The unit does slightly increase around the 50 mark though. The silver lining to triggering only at a higher polution count, is that you're not replacing your filter as often.
The biggest downside of these units, is that the filters have a unique RFID chip in then. Once the purifier decides the filter is "full", it forces you to buy a new filter, since you're unable to reuse the same filter. The filter's lifespan is solely based on the hours run (times by the fan speed). So if you ran you purifier in a 100% clean environment, you filter will still "wear out". Certainly a big drawback for this purifier, but most purifiers have gone this direction, so perhaps, good luck finding one that doesnt.
Overall, I'm very happy with the purifier. Would buy again.