I have had this unit for about a month now. Due to smoke from bushfires that reached us and a couple of smokers in the house, the filter is at about 85% left which seems pretty good to me since it has run every day.
It reacts instantly to smoke (from outside) or someone smoking inside.
It can be a bit noisy when display shows high levels of pollutants in the air, but not for too long because cleans them out quickly.
The app control is great! I can turn it on or off even when I am not at home. I can see what the air quality, temperature and humidity is even when I am not there.
I set up some automation so the unit turns on as soon as the air needs cleaning and turn off again when the job is done.
My doctor told me to get an air purifier and research brought me to this Xiaomi unit.
I would recommend this to anyone who, like me, has asthma.