I use this purifier mostly to keep the air in our bedroom clean. We heat our home with a wood-burning stove which creates dust and obviously some smoke when the door is opened. The unit has a sensor meant to detect certain contaminants in the air and when run on Auto mode, it automatically runs at a higher fan speed to clear the air faster. The colored glow from the bottom will change from green (good air quality) to yellow or red for bad air.
So far the auto mode has only kicked on once when I was cooking and generated some smoke. Mostly it just runs quietly in the background on the lowest fan speed and we often shut it off during the day. The fan on the lowest speed is so quiet you hardly notice it which is one of my requirements for any fan or purifier or humidifier. I don't like white noise at all. On the max setting it's noticeable noise, but there's no reason to run it on max unless you just burnt a quiche or something.
It's hard to say definitively that our air is cleaner but it's nice knowing that while we sleep, it's circulating the air and pulling it through the HEPA filter. We have two cats as well and while allergy season isn't here yet, we're hoping it helps my girlfriend's pollen allergy which gets really bad at certain times.
The only reason I didn't give it 5 stars is the air quality light indicator. The glow from the bottom is pretty bright and since it is green 99% of the time, I don't really see the point in such a bold indicator for the sensor. At night, you can put it in sleep mode which runs the fan on the lowest setting and turns the light out, but I would prefer the option to shut the light out entirely. I think some tiny led lights like they use on the control panel would be a much better option for the light indicator. It's not a dealbreaker but just seems unnecessary.
Otherwise the design is simple and clean, the unit isn't any bigger than it needs to be and it's powerful enough for our small bedroom which is all we needed in a purifier.