Having tried several cleaners in the category, the Xiaomi Smart Air Purifier 4 Pro is the winner, not so much for its capabilities (which are top-notch), but for the ease with which you can integrate it into a custom ecosystem.

The hardware:
The machine "flies." Its sensor is quite accurate and the fan is capable of handling large volumes of air. The size and quality of the filter is its strong point – it perhaps offers the best quality/price ratio on the market, translating the huge filtering surface into fewer changes and truly clean air.

Performance with smoke:
Here it really "works wonders." If you are a smoker, it is the ultimate tool. With the right setup, you can't tell at all that you smoked in the room after a while. The air quality is transformed to such a degree that the room smells as if it has just been well-ventilated, with no trace of cigarette smell.

The "Pro" tip (for the advanced users):
If you just take it out of the box and leave it on "Auto" via the Mi Home App, you lose 50% of its value. Because the purifier is in a different room from my office, the default auto-mode did not catch the smoke in time, while the "Favorite" mode is too noisy for continuous use.

The trick (Custom YAML Automation):
Don't limit yourself to the Cloud. Connect it to Home Assistant. I wrote custom YAML automations that "read" my own PM2.5 sensor (Sonoff) in the office and combine it with the purifier's internal sensor.

The logic: The purifier activates in Manual/Percentage: 100 only if the office exceeds 120 PM2.5 AND the purifier itself detects more than 30 PM2.5 in the living room, and includes a condition so that it does nothing if the fan is already at 100%. As soon as the pollution drops below 15, it automatically switches to Auto via a template condition.

With this setup, the machine remains silent 90% of the time, but becomes a "beast" only when it really needs to, without wasting filter and energy unnecessarily. If you have Home Assistant, it's a no-brainer. If not, get it for its build quality, but you'll miss all the magic of automation.

After one month of use, I can write my review to help future buyers!
I use it in a space of 40+ square meters, mainly for cigarette smoke.
Quite satisfied, its sensor detects the change in air directly within a few seconds of lighting a cigarette (in the same room) and after about a minute in an adjacent room.
What sets it apart and is very important is that its sensor for atmospheric pollution (pm 2.5) is set to European standards and not Chinese like some other models in the Xiaomi series.
For example, this particular device considers air quality good from 0 to 20 (pm 2.5) as defined by Europe, while in other models, the air is considered clean from 50 and above (pm 2.5) (China).

I will start with the negatives of the device and then move on to the positives.

Negatives:
- Size (assuming it needs about 10-15 centimeters of space around it and from furniture to take in the air). However, the positive aspect is that it is square-shaped and has height.
- Noise: At max, it makes noise, although I consider it normal since it is for 60 square meters but not annoying. In sleep mode, there are quieter machines, but I would not say it is bothersome (the noise can be adjusted as you will read below in the explanation of its application).
- It does not handle odors well (cooking, etc.), but it works great with cigarette smoke as it does not make the house smell like smoke at all. (Of course, if someone smokes heavily and a long time before purchasing the device, it is good to wash curtains, throws, carpets, and large surfaces as fabrics will smell, not the air. Personally, I washed everything before buying the device and for a month now, the house does not smell like smoke at all since the air is now cleaned directly and does not "stick" to fabrics and furniture anymore).

Positives:
+ Set to European air pollution standards (quite important)
+ The application is quite flexible to tailor it to our needs
+ Compatible with Alexa (I have not tested it with Google Home)
+ Quick response from the sensors and air purification
+ Dual-effect sensor for PM2.5 / PM10
+ Negative ionization of the air

Now, let's move on to the features of the application for those interested, as the application and capabilities vary from model to model:
- On / Off
- Operating mode settings:
Automatic (automatic speed according to the polluted air)
Sleep mode (lowest speed)
Manual (adjust the square meters of the space and it operates at a constant speed)
3 manual speeds: Constant low / constant medium / constant high

- Negative ionization of the air: On / Off (refreshes the air as if we have ventilated the house)
- Filter indicator showing how many days are left until replacement
- Automatic opening and closing at specific times and days
- Screen brightness adjustment: Off / low / high
- Child lock to prevent buttons from being pressed on the machine (touch screen)
- Machine sounds: On / Off
- Smart scenes: Create various scenarios for the device to operate automatically. However, there is a delay in the scenarios as they are done via the internet through a server in China, which, although they work correctly, the delay is significant and did not suit me. GENERAL:

ATTENTION: When installing the application on your mobile (Xiaomi Home), it will ask you from the beginning in which country (server) you are. There, we select Singapore and not Europe or Greece in order for the application to work smoothly.
In case for some reason you mess up during the installation, the device can be reset as follows: With the device turned on (essential), press the small white button on the back for 7 seconds until you hear a beep.

The connection with Alexa was automatically made (pay attention to the application as we mentioned, select Singapore).
It is convenient and its operation with Alexa connection, as we can create scenarios through it (Personally, I set it up so that when I leave home, it opens the device for an hour to clean while I'm away and automatically closes and opens when I return home).

Sorry if I bored you with my review, I just wanted to be as thorough as possible to help someone who is interested in buying it because it is not a cheap purchase to buy blindly.