I've been using this for a few days now. When I replace the bedding in the room with the filter, it will immediately sense dust or something in the air and crank up for a little bit. The changes are nearly instantaneous. Which is good. Or sometimes not so good when it ramps up, then down, then up, then down making the (very quiet)  noise a bit more noticeable.  

The LED showing the air quality is very helpful. It's a bit too bright for a bedroom at night, but a little strip of post-it note covering it dims it sufficiently and you can still see it shine through in the day. 

I did notice it ramp up one day, and was surprised -- what I didn't notice is that air quality in my neighborhood had gotten significantly worse. Having that automatic feature gives me peace of mind, and I no longer have to obsessively check PurpleAir maps.